Liquid Crystal Display Device and Driving Method Thereof

ABSTRACT

To provide a hold-type display device without a problem of motion blur and a driving method thereof. The length of a period for displaying a blanking image in one frame period is controlled in accordance with a control parameter showing the degree of motion blur, and the level of a signal supplied to a display element is changed in accordance with the length of the period for displaying the blanking image. Accordingly, the hold-type display device without a problem of motion blur and the driving method thereof can be provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display device and an operatingmethod of the display device. In particular, the present inventionrelates to a method for improving quality of a moving image of ahold-type display device.

2. Description of the Related Art

In recent years, there have been growing interests in thin displaydevices. Liquid crystal displays, plasma displays, projection displays,and the like have been developed and becoming popular instead of CRTdisplays. Further, field emission displays, inorganicelectroluminescence displays, organic electroluminescence displays,electronic paper, and the like have been developed as next-generationdisplay devices.

In a display portion which is provided in the above-described displaydevice, pixels which are minimum units for forming an image arearranged. Each of the pixels emits light with desired luminance by beingsupplied with a signal generated by image data. Accordingly, an image isdisplayed in the display portion.

In addition, the signal supplied to the pixel is updated (refreshed) ata constant period. An inverse number of this period is referred to as aframe rate. Further, time after the signal is updated once and beforethe signal is updated next is referred to as one frame period. Displayof a moving image in the display portion is realized by supplying asignal which is different from the signal supplied before to the pixelwhen the signal is updated. On the other hand, display of a still imagein the display portion is realized by supplying a signal which is thesame as the signal supplied before to the pixel when the signal isupdated.

Further, driving methods of display devices can be classified bytemporal distribution of luminance of a pixel in one frame period. Inhold-type display devices which are used for active matrix displaydevices, a pixel continuously emits light in one frame period. On theother hand, in impulsive-type display devices typified by CRTs, a pixelimmediately attenuates and does not emit light after the pixel stronglyemits light once in one frame period. In impulsive-type display devices,most one frame period is a non-light emitting state.

It has become obvious that hold-type display devices have a problem suchthat a moving object seems to leave traces when a moving image isdisplayed and part of an image is moved or the entire image blurs whenthe entire image is moved (motion blur). This is characteristics ofhold-type display devices and a problem of motion blur does not occur inimpulsive-type display devices.

As a method for solving a problem of motion blur in hold-type displaydevices, the following two methods have been mainly proposed (seeReference 1). A first method is a method of providing a period duringwhich the original image is displayed and a period during which a blackimage is displayed in one frame period. Thus, display can be made closerto that of impulsive-type display devices, so that quality of a movingimage can be improved (see References 2 and 3). A second method is amethod in which display is performed by shortening the length of oneframe period (increasing a frame rate) and generating a temporallycompensated image with respect to an increased frame at the same time.Thus, quality of a moving image can be improved (see Reference 4). Inaddition, as an improvement technology of the first method, it isdisclosed that quality of a moving image can be improved by displaying adarker image than the original image instead of a black image (seeReferences 5, 6, 9, 10, and 11). Further, a method of changing drivingmethods in accordance with conditions is also disclosed (see Reference 7and 8).

-   [Reference 1] Japanese Published Patent Application No. H04-302289-   [Reference 2] Japanese Published Patent Application No. H09-325715-   [Reference 3] Japanese Published Patent Application No. 2000-200063-   [Reference 4] Japanese Published Patent Application No. 2005-268912-   [Reference 5] Japanese Published Patent Application No. 2002-23707-   [Reference 6] Japanese Published Patent Application No. 2004-240317-   [Reference 7] Japanese Published Patent Application No. 2002-91400-   [Reference 8] Japanese Published Patent Application No. 2004-177575-   [Reference 9] Society For Information Display '05 DIGEST, 60.2, pp.    1734 to 1737, (2005)-   [Reference 10] Society For Information Display '06 DIGEST, 69.4, pp.    1950 to 1953, (2006)-   [Reference 11] Society For Information Display '06 DIGEST, 69.5, pp.    1954 to 1957, (2006)

SUMMARY OF THE INVENTION

Although various methods for solving a problem of motion blur inhold-type display devices have been considered, advantageous effectsthereof are not sufficient in some cases. In addition, other troublesare increased by using the methods in some cases. For example, a flickerincreases in a method in which display is made closer to that ofimpulsive-type display devices by displaying a black image. In addition,by displaying the black image, luminance of an image decreases comparedwith the case of not inserting a black image. In that case, in order toobtain luminance which is equal to that of the case of not inserting ablack image, it is necessary to increase luminance instantaneously.Accordingly, loads on display devices are increased to decreasereliability or increase power consumption, which becomes problematic.

In a method of increasing a frame rate, a driver circuit which canprocess data at high speed is necessary because data processing becomescomplicated, so that manufacturing cost increases, heat is generated inaccordance with data processing, and power consumption increases, whichbecome problematic. In addition, in a method in which a new image isgenerated by compensating image data, it is difficult to obtainhigh-quality compensated image, and on the contrary, quality of a movingimage decreases in some cases by displaying an image by insertion ofcompensated data.

Further, when the above-described method for solving a problem of motionblur is applied to a liquid crystal display device, there is a problemin that speed during which transmittance of a liquid crystal is changeis slow and it is difficult to sufficiently follow change in signals.Furthermore, there is a problem in that degree of light emission of apixel is changed depending on viewing angles.

The present invention has been made in view of the foregoing problems.It is an object of the present invention to provide a hold-type displaydevice without a problem of motion blur and a driving method of thereof.It is another object of the present invention to provide a displaydevice with low power consumption and a driving method of thereof. Inaddition, it is another object of the present invention to provide adisplay device with improved quality for still images and moving imagesand a driving method of thereof. Further, it is another object of thepresent invention to provide a display device with a wider viewing angleand a driving method of thereof. Furthermore, it is an object of thepresent invention to provide a display device with improved responsespeed of a liquid crystal and a driving method of thereof.

One aspect of the present invention is a driving method of a liquidcrystal display device in which an image is displayed by applying signalvoltage V_(i) in accordance with an image signal to a liquid crystalelement. One frame period is divided into a first subframe period and asecond subframe period. When the length of the first subframe period isdenoted by τ_(a), first voltage which is applied to the liquid crystalelement in the first subframe period is denoted by V_(a), and secondvoltage which is applied to the liquid crystal element in the secondsubframe period is denoted by V_(b), the first voltage V_(a) isdetermined in accordance with a difference between the second voltageV_(b) and the signal voltage V_(i), and the length of the first subframeperiod τ_(a). The second voltage V_(b) is voltage at which the liquidcrystal element performs black display.

Another aspect of the present invention is a driving method of a liquidcrystal display device in which an image is displayed by applying signalvoltage V_(i) in accordance with an image signal to a liquid crystalelement. The liquid crystal display device includes a backlight. Whenthe length of a backlight lighting period in one frame period is denotedby τ_(a), first voltage which is applied to the liquid crystal elementin one frame period is denoted by V_(a), and initialization voltagewhich is applied to the liquid crystal element right before one frameperiod is denoted by V₀, the first voltage V_(a) is determined inaccordance with a difference between the initialization voltage V₀ andthe signal voltage V_(i), and the length of the backlight lightingperiod τ_(a).

Another aspect of the present invention is a driving method of a liquidcrystal display device in which an image is displayed by applying signalvoltage V_(i) in accordance with an image signal to a liquid crystalelement. The liquid crystal display device includes a backlight. Thebacklight is divided into a plurality of light-emitting regions in adisplay region and is sequentially scanned to emit light. When thelength of a lighting period of each of a plurality of light-emittingregions in one frame period is denoted by τ_(a), first voltage which isapplied to the liquid crystal element in one frame period is denoted byV_(a), and initialization voltage which is applied to the liquid crystalelement right before one frame period is denoted by V₀, the firstvoltage V_(a) is determined in accordance with a difference between theinitialization voltage V₀ and the signal voltage V_(i), and the lengthof the lighting period τ_(a) of each of the plurality of light-emittingregions.

Another aspect of the present invention is a driving method of a liquidcrystal display device in which an image is displayed by applying signalvoltage V_(i) in accordance with an image signal to a liquid crystalelement. The liquid crystal display device includes a backlight. Oneframe period is divided into a first subframe period and a secondsubframe period. When the length of a backlight lighting period in oneframe period is denoted by τ_(a1) the length of the first subframeperiod is denoted by τ_(a2), first voltage which is applied to theliquid crystal element in the first subframe period is denoted by V_(a),and second voltage which is applied to the liquid crystal element in thesecond subframe period is denoted by V_(b), the first voltage V_(a) isdetermined in accordance with a difference between the second voltageV_(b) and the signal voltage V_(i), the length of the backlight lightingperiod Σ_(a1) and the length of the first subframe period τ_(a2). Thesecond voltage V_(b) is voltage at which the liquid crystal elementperforms black display.

Another aspect of the present invention is a driving method of a liquidcrystal display device in which an image is displayed by applying signalvoltage V_(i) in accordance with an image signal to a liquid crystalelement. The liquid crystal display device includes a backlight. Thebacklight is divided into a plurality of light-emitting regions in adisplay region and is sequentially scanned to emit light. One frameperiod is divided into a first subframe period and a second subframeperiod. When the length of a lighting period of each of a plurality oflight-emitting regions in one frame period is denoted by τ_(a1), thelength of the first subframe period is denoted by τ_(a2), first voltagewhich is applied to the liquid crystal element in the first subframeperiod is denoted by V_(a), and second voltage which is applied to theliquid crystal element in the second subframe period is denoted byV_(b), the first voltage V_(a) is determined in accordance with adifference between the second voltage V_(b), and the signal voltage thelength of the lighting period τ_(a1) of each of the plurality oflight-emitting regions, and the length of the first subframe periodτ_(a2). The second voltage V_(b) is voltage at which the liquid crystalelement performs black display.

Another aspect of the present invention is a liquid crystal displaydevice in which an image is displayed by applying signal voltage V_(i)in accordance with an image signal to a liquid crystal element. Oneframe period is divided into a first subframe period and a secondsubframe period. When the length of the first subframe period is denotedby τ_(a), first voltage which is applied to the liquid crystal elementin the first subframe period is denoted by V_(a), and second voltagewhich is applied to the liquid crystal element in the second subframeperiod is denoted by V_(b), the first voltage V_(a) is determined inaccordance with a difference between the second voltage V_(b) and thesignal voltage V_(i), and the length of the first subframe period τ_(a).The second voltage V_(b) is voltage at which the liquid crystal elementperforms black display.

Another aspect of the present invention is a liquid crystal displaydevice in which an image is displayed by applying signal voltage V_(i)in accordance with an image signal to a liquid crystal element. Theliquid crystal display device includes a backlight. When the length of abacklight lighting period in one frame period is denoted by τ_(a), firstvoltage which is applied to the liquid crystal element in one frameperiod is denoted by V_(a), and initialization voltage which is appliedto the liquid crystal element right before one frame period is denotedby V₀, the first voltage V_(a) is determined in accordance with adifference between the initialization voltage V₀ and the signal voltageV_(i), and the length of the backlight lighting period τ_(a).

Another aspect of the present invention is a liquid crystal displaydevice in which an image is displayed by applying signal voltage V_(i)in accordance with an image signal to a liquid crystal element. Theliquid crystal display device includes a backlight. The backlight isdivided into a plurality of light-emitting regions in a display regionand is sequentially scanned to emit light. When the length of a lightingperiod of each of a plurality of light-emitting regions in one frameperiod is denoted by τ_(a), first voltage which is applied to the liquidcrystal element in one frame period is denoted by V_(a), andinitialization voltage which is applied to the liquid crystal elementright before one frame period is denoted by V₀, the first voltage V_(a)is determined in accordance with a difference between the initializationvoltage V₀ and the signal voltage V_(i), and the length of the lightingperiod τ_(a) of each of the plurality of light-emitting regions.

Another aspect of the present invention is a liquid crystal displaydevice in which an image is displayed by applying signal voltage V_(i)in accordance with an image signal to a liquid crystal element. Theliquid crystal display device includes a backlight. One frame period isdivided into a first subframe period and a second subframe period. Whenthe length of a backlight lighting period in one frame period is denotedby τ_(a1), the length of the first subframe period is denoted by τ_(a2),first voltage which is applied to the liquid crystal element in thefirst subframe period is denoted by V_(a), and second voltage which isapplied to the liquid crystal element in the second subframe period isdenoted by V_(b), the first voltage V_(a) is determined in accordancewith a difference between the second voltage V_(b) and the signalvoltage V_(i), the length of the backlight lighting period τ_(a1), andthe length of the first subframe period τ_(a2). The second voltage V_(b)is voltage at which the liquid crystal element performs black display.

Another aspect of the present invention is a liquid crystal displaydevice in which an image is displayed by applying signal voltage V_(i)in accordance with an image signal to a liquid crystal element. Theliquid crystal display device includes a backlight. The backlight isdivided into a plurality of light-emitting regions in a display regionand is sequentially scanned to emit light. One frame period is dividedinto a first subframe period and a second subframe period. When thelength of a lighting period of each of a plurality of light-emittingregions in one frame period is denoted by τ_(a1) the length of the firstsubframe period is denoted by τ_(a2), first voltage which is applied tothe liquid crystal element in the first subframe period is denoted byV_(a), and second voltage which is applied to the liquid crystal elementin the second subframe period is denoted by V_(b), the first voltageV_(a) is determined in accordance with a difference between the secondvoltage V_(b) and the signal voltage V_(i), the length of the lightingperiod τ_(a1) of each of the plurality of light-emitting regions, andthe length of the first subframe period Σ_(a2). The second voltage V_(b)is voltage at which the liquid crystal element performs black display.

Note that in this specification, a condition where the darkest grayscale among gray scales which can be displayed is displayed even aslight amount of light is emitted is described that “luminance is 0” insome cases in addition to a condition where light is not emitted at all.

Note that various types of switches can be used as a switch shown inthis document (a specification, a claim, a drawing, and the like). Anelectrical switch, a mechanical switch, and the like are given asexamples. That is, any element can be used as long as it can control acurrent flow, without limiting to a certain element. For example, atransistor (e.g., a bipolar transistor or a MOS transistor), a diode(e.g., a PN diode, a PIN diode, a Schottky diode, a MIM (metal insulatormetal) diode, a MIS (metal insulator semiconductor) diode, or adiode-connected transistor), a thyristor, or the like can be used as aswitch. Alternatively, a logic circuit in which such elements arecombined can be used as a switch.

In the case of using a transistor as a switch, polarity (a conductivitytype) of the transistor is not particularly limited because it operatesjust as a switch. However, a transistor of polarity with smalleroff-current is preferably used when off-current is to be suppressed. Atransistor provided with an LDD region, a transistor with a multi-gatestructure, and the like are given as examples of a transistor withsmaller off-current. In addition, it is preferable that an N-channeltransistor be used when a potential of a source terminal of thetransistor which is operated as a switch is closer to a potential of alow-potential-side power supply (e.g., Vss, GND, or 0 V), while aP-channel transistor be used when the potential of the source terminalis closer to a potential of a high-potential-side power supply (e.g.,Vdd). This is because the absolute value of gate-source voltage can beincreased when the potential of the source terminal of the transistorwhich is operated as the switch is closer to a potential of alow-potential-side power supply in an N-channel transistor and when thepotential of the source terminal of the transistor which is operated asthe switch is closer to a potential of a high-potential-side powersupply in a P-channel transistor, so that the transistor is useful to beoperated as a switch. This is also because the transistor does not oftenperform a source follower operation, so that reduction in output voltagedoes not often occur.

Note that a CMOS switch using both N-channel and P-channel transistorsmay be used. By using a CMOS switch, the switch can easily operate as aswitch because current can flow when the P-channel transistor or theN-channel transistor is turned on. For example, voltage can beappropriately output regardless of whether voltage of an input signal ofthe switch is high or low. In addition, since a voltage amplitude valueof a signal for turning on or off the switch can be made small, powerconsumption can be reduced.

Note also that when a transistor is used as a switch, the switchincludes an input terminal (one of a source terminal and a drainterminal), an output terminal (the other of the source terminal and thedrain terminal), and a terminal for controlling electrical conduction (agate electrode). On the other hand, when a diode is used as a switch,the switch does not have a terminal for controlling electricalconduction in some cases. Therefore, when a diode is used as a switch,the number of wirings for controlling terminals can be more reduced thanthe case of using a transistor as a switch.

Note that in this document (the specification, the claim, the drawing,and the like), when it is explicitly described that “A and B areconnected”, the case where elements are electrically connected, the casewhere elements are functionally connected, and the case where elementsare directly connected are included therein. Here, each of A and Bcorresponds to an object (e.g., a device, an element, a circuit, awiring, an electrode, a terminal, a conductive film, or a layer).Accordingly, in structures disclosed in this document (thespecification, the claim, the drawing, and the like), another elementmay be interposed between elements having a connection relation shown indrawings and texts, without limiting to a predetermined connectionrelation, for example, the connection relation shown in the drawings andthe texts.

For example, in the case where A and B are electrically connected, oneor more elements which enable electrical connection of A and B (e.g., aswitch, a transistor, a capacitor, an inductor, a resistor, and/or adiode) may be provided between A and B. In addition, in the case where Aand B are functionally connected, one or more circuits which enablefunctional connection of A and B (e.g., a logic circuit such as aninverter, a NAND circuit, or a NOR circuit, a signal converter circuitsuch as a DA converter circuit, an AD converter circuit, or a gammacorrection circuit, a potential level converter circuit such as a powersupply circuit (e.g., a boosting circuit or a voltage lower controlcircuit) or a level shifter circuit for changing a potential level of asignal, a voltage source, a current source, a switching circuit, or anamplifier circuit such as a circuit which can increase signal amplitude,the amount of current, or the like (e.g., an operational amplifier, adifferential amplifier circuit, a source follower circuit, or a buffercircuit), a signal generating circuit, a memory circuit, and/or acontrol circuit) may be provided between A and B. Alternatively, in thecase where A and B are directly connected, A and B may be directlyconnected without interposing another element or another circuittherebetween.

Note that when it is explicitly described that “A and B are directlyconnected”, the case where A and B are directly connected (i.e., thecase where A and B are connected without interposing another element oranother circuit therebetween) and the case where A and B areelectrically connected (i.e., the case where A and B are connected byinterposing another element or another circuit therebetween) areincluded therein.

Note that when it is explicitly described that “A and B are electricallyconnected”, the case where A and B are electrically connected (i.e., thecase where A and B are connected by interposing another element oranother circuit therebetween), the case where A and B are functionallyconnected (i.e., the case where A and B are functionally connected byinterposing another circuit therebetween), and the case where A and Bare directly connected (i.e., the case where A and B are connectedwithout interposing another element or another circuit therebetween) areincluded therein. That is, when it is explicitly described that “A and Bare electrically connected”, the description is the same as the casewhere it is explicitly only described that “A and B are connected”.

Note that a display element, a display device which is a device having adisplay element, a light-emitting element, and a light-emitting devicewhich is a device having a light-emitting element can use various typesand can include various elements. For example, as a display element, adisplay device, a light-emitting element, and a light-emitting device,whose a display medium, contrast, luminance, reflectivity,transmittivity, or the like changes by an electromagnetic action, suchas an EL element (e.g., an EL element including organic and inorganicmaterials, an organic EL element, or an inorganic EL element), anelectron emitter, a liquid crystal element, electronic ink, anelectrophoresis element, a grating light valve (GLV), a plasma displaypanel (PDP), a digital micromirror device (DMD), a piezoelectric ceramicdisplay, or a carbon nanotube can be used. Note that display devicesusing an EL element include an EL display; display devices using anelectron emitter include a field emission display (FED), an SED-typeflat panel display (SED: Surface-conduction Electron-emitter Display),and the like; display devices using a liquid crystal element include aliquid crystal display (e.g., a transmissive liquid crystal display, asemi-transmissive liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display); and display devices using electronic ink includeelectronic paper.

Note that by using a catalyst (e.g., nickel) in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be formed. Atthis time, crystallinity can be improved by performing heat treatmentwithout using a laser. Accordingly, a gate driver circuit (e.g., a scanline driver circuit) and part of a source driver circuit (e.g., ananalog switch) can be formed over the same substrate. In addition, inthe case of not using a laser for crystallization, crystallinityunevenness of silicon can be suppressed. Therefore, an image having highquality can be displayed.

Note also that polycrystalline silicon and microcrystalline silicon canbe formed without using a catalyst (e.g., nickel).

In addition, a transistor can be formed by using a semiconductorsubstrate, an SOI substrate, or the like. In that case, a MOStransistor, a junction transistor, a bipolar transistor, or the like canbe used as a transistor described in this specification. Therefore, atransistor with few variations in characteristics, sizes, shapes, or thelike, with high current supply capacity, and with a small size can beformed. By using such a transistor, power consumption of a circuit canbe reduced or a circuit can be highly integrated.

In addition, a transistor including a compound semiconductor or a oxidesemiconductor such as ZnO, a-InGaZnO, SiGe, GaAs, IZO, indium tin oxide(ITO), or SnO, and a thin film transistor or the like obtained bythinning such a compound semiconductor or a oxide semiconductor can beused. Therefore, manufacturing temperature can be lowered and forexample, such a transistor can be formed at room temperature.Accordingly, the transistor can be formed directly on a substrate havinglow heat resistance such as a plastic substrate or a film substrate.Note that such a compound semiconductor or an oxide semiconductor can beused for not only a channel portion of the transistor but also otherapplications. For example, such a compound semiconductor or an oxidesemiconductor can be used as a resistor, a pixel electrode, or alight-transmitting electrode. Further, since such an element can beformed at the same time as the transistor, cost can be reduced.

A transistor or the like formed by using an inkjet method or a printingmethod can also be used. Accordingly, a transistor can be formed at roomtemperature, can be formed at a low vacuum, or can be formed using alarge substrate. In addition, since the transistor can be formed withoutusing a mask (a reticle), layout of the transistor can be easilychanged. Further, since it is not necessary to use a resist, materialcost is reduced and the number of steps can be reduced. Furthermore,since a film is formed only in a necessary portion, a material is notwasted compared with a manufacturing method in which etching isperformed after the film is formed over the entire surface, so that costcan be reduced.

Further, a transistor or the like including an organic semiconductor ora carbon nanotube can be used. Accordingly, such a transistor can beformed using a substrate which can be bent. Therefore, a device using atransistor or the like including an organic semiconductor or a carbonnanotube can resist a shock.

Furthermore, transistors with various structures can be used. Forexample, a MOS transistor, a junction transistor, a bipolar transistor,or the like can be used as a transistor described in this document (thespecification, the claim, the drawing, and the like). By using a MOStransistor, the size of the transistor can be reduced. Thus, a pluralityof transistors can be mounted. By using a bipolar transistor, largecurrent can flow. Thus, a circuit can be operated at high speed.

A MOS transistor, a bipolar transistor, and the like may be formed overone substrate. Thus, reduction in power consumption, reduction in size,high speed operation, and the like can be realized.

Furthermore, various transistors can be used.

A transistor can be formed using various types of substrates. The typeof a substrate where a transistor is formed is not limited to a certaintype. For example, a single crystalline substrate, an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a papersubstrate, a cellophane substrate, a stone substrate, a wood substrate,a cloth substrate (including a natural fiber (e.g., silk, cotton, orhemp), a synthetic fiber (e.g., nylon, polyurethane, or polyester), aregenerated fiber (e.g., acetate, cupra, rayon, or regeneratedpolyester), or the like), a leather substrate, a rubber substrate, astainless steel substrate, a substrate including a stainless steel foil,or the like can be used as a substrate where the transistor is formed.Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human being can be used as a substrate where thetransistor is formed. In addition, the transistor may be formed usingone substrate, and then, the transistor may be transferred to anothersubstrate. A single crystalline substrate, an SOI substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a paper substrate, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester), or thelike), a leather substrate, a rubber substrate, a stainless steelsubstrate, a substrate including a stainless steel foil, or the like canbe used as a substrate to which the transistor is transferred.Alternatively, a skin (e.g., epidermis or corium) or hypodermal tissueof an animal such as a human being can be used as a substrate to whichthe transistor is transferred. Further alternatively, the transistor maybe formed using one substrate and the substrate may be thinned bypolishing. A single crystalline substrate, an SOI substrate, a glasssubstrate, a quartz substrate, a plastic substrate, a paper substrate, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester), or thelike), a leather substrate, a rubber substrate, a stainless steelsubstrate, a substrate including a stainless steel foil, or the like canbe used as a substrate to be polished. Alternatively, a skin (e.g.,epidermis or corium) or hypodermal tissue of an animal such as a humanbeing can be used as a substrate to be polished. By using such asubstrate, a transistor with excellent properties or a transistor withlow power consumption can be formed, a device with high durability orhigh heat resistance can be formed, or reduction in weight or thicknesscan be achieved.

A structure of a transistor can be various modes without limiting to acertain structure. For example, a multi-gate structure having two ormore gate electrodes may be used. When the multi-gate structure is used,a structure where a plurality of transistors are connected in series isprovided because a structure where channel regions are connected inseries is provided. By using the multi-gate structure, off-current canbe reduced or the withstand voltage of the transistor can be increasedto improve reliability. Alternatively, by using the multi-gatestructure, drain-source current does not fluctuate very much even ifdrain-source voltage fluctuates when the transistor operates in asaturation region, so that a flat slope of voltage-currentcharacteristics can be obtained. By utilizing the flat slope of thevoltage-current characteristics, an ideal current source circuit or anactive load having a high resistance value can be realized. Accordingly,a differential circuit or a current mirror circuit having excellentproperties can be realized. In addition, a structure where gateelectrodes are formed above and below a channel may be used. By usingthe structure where gate electrodes are formed above and below thechannel, a channel region is enlarged, so that the amount of currentflowing therethrough can be increased or a depletion layer can be easilyformed to decrease an S value. When the gate electrodes are formed aboveand below the channel, a structure where a plurality of transistors areconnected in parallel is provided.

Further, a structure where a gate electrode is formed above a channel, astructure where a gate electrode is formed below a channel, a staggeredstructure, an inversely staggered structure, a structure where a channelregion is divided into a plurality of regions, or a structure wherechannel regions are connected in parallel or in series can be used. Inaddition, a source electrode or a drain electrode may overlap with achannel region (or part of it). By using the structure where the sourceelectrode or the drain electrode may overlap with the channel region (orpart of it), the case can be prevented in which electric charges areaccumulated in part of the channel region, which would result in anunstable operation. Further, an LDD region may be provided. By providingthe LDD region, off-current can be reduced or the withstand voltage ofthe transistor can be increased to improve reliability. Alternatively,drain-source current does not fluctuate very much even if drain-sourcevoltage fluctuates when the transistor operates in the saturationregion, so that a flat slope of voltage-current characteristics can beobtained.

Note that various types of transistors can be used for a transistor inthis document (the specification, the claim, the drawing, and the like)and the transistor can be formed using various types of substrates.Accordingly, all of circuits which are necessary to realize apredetermined function may be formed using the same substrate. Forexample, all of the circuits which are necessary to realize thepredetermined function may be formed using a glass substrate, a plasticsubstrate, a single crystalline substrate, an SOI substrate, or anyother substrate. When all of the circuits which are necessary to realizethe predetermined function are formed using the same substrate, cost canbe reduced by reduction in the number of component parts or reliabilitycan be improved by reduction in the number of connections to circuitcomponents. Alternatively, part of the circuits which are necessary torealize the predetermined function may be formed using one substrate andanother part of the circuits which are necessary to realize thepredetermined function may be formed using another substrate. That is,not all of the circuits which are necessary to realize the predeterminedfunction are required to be formed using the same substrate. Forexample, part of the circuits which are necessary to realize thepredetermined function may be formed with transistors using a glasssubstrate and another part of the circuits which are necessary torealize the predetermined function may be formed using a singlecrystalline substrate, so that an IC chip formed by a transistor usingthe single crystalline substrate may be connected to the glass substrateby COG (chip on glass) and the IC chip may be provided over the glasssubstrate. Alternatively, the IC chip may be connected to the glasssubstrate by TAB (tape automated bonding) or a printed wiring board.When part of the circuits are formed using the same substrate in thismanner, cost can be reduced by reduction in the number of componentparts or reliability can be improved by reduction in the number ofconnections to circuit components. In addition, for example, by forminga portion with high driving voltage or a portion with high drivingfrequency, which consumes large power, using a single crystallinesubstrate and using an IC chip formed by the circuit instead of formingsuch a portion using the same substrate, increase in power consumptioncan be prevented.

Note also that one pixel corresponds to one element whose brightness canbe controlled in this document (the specification, the claim, thedrawing, and the like). For example, one pixel corresponds to one colorelement which expresses brightness. Therefore, in the case of a colordisplay device having color elements of R (Red), G (Green), and B(Blue), a minimum unit of an image is formed of three pixels of an Rpixel, a G pixel, and a B pixel. Note that the color elements are notlimited to three colors, and color elements of more than three colorsmay be used or a color other than RGB may be added. For example, RGBWmay be used by adding W (white). In addition, RGB plus one or morecolors of yellow, cyan, magenta emerald green, vermilion, and the likemay be used. Further, a color similar to at least one of R, G, and B maybe added to RGB. For example, R, G, B1, and B2 may be used. Althoughboth B1 and B2 are blue, they have slightly different frequency.Similarly, R1, R2, G, and B may be used, for example. By using suchcolor elements, display which is closer to the real object can beperformed. Alternatively, by using such color elements, powerconsumption can be reduced. Furthermore, as another example, in the caseof controlling brightness of one color element by using a plurality ofregions, one region may correspond to one pixel. For example, in thecase of performing area ratio gray scale display or in the case ofincluding a subpixel, a plurality of regions which control brightnessare provided in each color element and gray scales are expressed withthe whole regions. In this case, one region which controls brightnessmay correspond to one pixel. Thus, in that case, one color elementincludes a plurality of pixels. Alternatively, even when the pluralityof regions which control brightness are provided in one color element,these regions may be collected as one pixel. Thus, in that case, onecolor element includes one pixel. In that case, one color elementincludes one pixel. In the case where brightness is controlled in aplurality of regions in each color element, regions which contribute todisplay have different area dimensions depending on pixels in somecases. In addition, in the plurality of regions which control brightnessin each color element, signals supplied to each of the plurality ofregions may be slightly varied to widen a viewing angle. That is,potentials of pixel electrodes included in the plurality of regionsprovided in each color element may be different from each other.Accordingly, voltage applied to liquid crystal molecules are varieddepending on the pixel electrodes. Therefore, the viewing angle can bewidened.

Note that when it is explicitly described that “one pixel (for threecolors)”, it corresponds to the case where three pixels of R, and B areconsidered as one pixel. Meanwhile, when it is explicitly described that“one pixel (for one color)”, it corresponds to the case where theplurality of regions are provided in each color element and collectivelyconsidered as one pixel.

Note also that in this document (the specification, the claim, thedrawing, and the like), pixels are provided (arranged) in matrix in somecases. Here, description that pixels are provided (arranged) in matrixincludes the case where the pixels are arranged in a straight line andthe case where the pixels are arranged in a jagged line, in alongitudinal direction or a lateral direction. Therefore, in the case ofperforming full color display with three color elements (e.g., RGB), thefollowing cases are included therein: the case where the pixels arearranged in stripes and the case where dots of the three color elementsare arranged in a delta pattern. In addition, the case is also includedtherein in which dots of the three color elements are provided in Bayerarrangement. Note that the color elements are not limited to threecolors, and color elements of more than three colors may be used. RGBW,RGB plus one or more of yellow, cyan, magenta, and the like, or the likeis given as an example. Further, the sizes of display regions may bedifferent between respective dots of color elements. Thus, powerconsumption can be reduced or the life of a display element can beprolonged.

Note also that in this document (the specification, the claim, thedrawing, and the like), an active matrix method in which an activeelement is included in a pixel or a passive matrix method in which anactive element is not included in a pixel can be used.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also various active elements(non-linear elements) can be used. For example, a MIM (metal insulatormetal), a TFD (thin film diode), or the like can also be used. Sincesuch an element has few number of manufacturing steps, manufacturingcost can be reduced or yield can be improved. Further, since the size ofthe element is small, an aperture ratio can be improved, so that powerconsumption can be reduced or high luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, manufacturing steps is few, so that manufacturing cost can bereduced or the yield can be improved. Further, since an active element(a non-linear element) is not used, the aperture ratio can be improved,so that power consumption can be reduced or high luminance can beachieved.

Note that a transistor is an element having at least three terminals ofa gate, a drain, and a source. The transistor has a channel regionbetween a drain region and a source region, and current can flow throughthe drain region, the channel region, and the source region. Here, sincethe source and the drain of the transistor may change depending on thestructure, the operating condition, and the like of the transistor, itis difficult to define which is a source or a drain. Therefore, in thisdocument (the specification, the claim, the drawing, and the like), aregion functioning as a source and a drain may not be called the sourceor the drain. In such a case, for example, one of the source and thedrain may be referred to as a first terminal and the other thereof maybe referred to as a second terminal. Alternatively, one of the sourceand the drain may be referred to as a first electrode and the otherthereof may be referred to as a second electrode. Further alternatively,one of the source and the drain may be referred to as a source regionand the other thereof may be called a drain region.

Note also that a transistor may be an element having at least threeterminals of a base, an emitter, and a collector. In this case also, oneof the emitter and the collector may be similarly called a firstterminal and the other terminal may be called a second terminal.

A gate corresponds to all or part of a gate electrode and a gate wiring(also referred to as a gate line, a gate signal line, a scan line, ascan signal line, or the like). A gate electrode corresponds to aconductive film which overlaps with a semiconductor which forms achannel region with a gate insulating film interposed therebetween. Notethat part of the gate electrode overlaps with an LDD (lightly dopeddrain) region, the source region, or the drain region with the gateinsulating film interposed therebetween in some cases. A gate wiringcorresponds to a wiring for connecting a gate electrode of eachtransistor to each other, a wiring for connecting a gate electrode ofeach pixel to each other, or a wiring for connecting a gate electrode toanother wiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) which functions as both a gate electrode and a gate wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a gate electrode or a gate wiring. That is, there is aregion where a gate electrode and a gate wiring cannot be clearlydistinguished from each other. For example, in the case where a channelregion overlaps with part of an extended gate wiring, the overlappedportion (region, conductive film, wiring, or the like) functions as botha gate wiring and a gate electrode. Accordingly, such a portion (aregion, a conductive film, a wiring, or the like) may be called either agate electrode or a gate wiring.

In addition, a portion (a region, a conductive film, a wiring, or thelike) which is formed of the same material as a gate electrode, formsthe same island as the gate electrode, and is connected to the gateelectrode may also be called a gate electrode. Similarly, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate wiring, forms the same island as the gatewiring, and is connected to the gate wiring may also be called a gatewiring. In a strict sense, such a portion (a region, a conductive film,a wiring, or the like) does not overlap with a channel region or doesnot have a function of connecting the gate electrode to another gateelectrode in some cases. However, there is a portion (a region, aconductive film, a wiring, or the like) which is formed of the samematerial as a gate electrode or a gate wiring, forms the same island asthe gate electrode or the gate wiring, and is connected to the gateelectrode or the gate wiring because of conditions in a manufacturingstep. Thus, such a portion (a region, a conductive film, a wiring, orthe like) may also be called either a gate electrode or a gate wiring.

In a multi-gate transistor, for example, a gate electrode is oftenconnected to another gate electrode by using a conductive film which isformed of the same material as the gate electrode. Since such a portion(a region, a conductive film, a wiring, or the like) is a portion (aregion, a conductive film, a wiring, or the like) for connecting thegate electrode to another gate electrode, it may be called a gatewiring, and it may also be called a gate electrode because a multi-gatetransistor can be considered as one transistor. That is, a portion (aregion, a conductive film, a wiring, or the like) which is formed of thesame material as a gate electrode or a gate wiring, forms the sameisland as the gate electrode or the gate wiring, and is connected to thegate electrode or the gate wiring may be called either a gate electrodeor a gate wiring. In addition, for example, part of a conductive filmwhich connects the gate electrode and the gate wiring and is formed of amaterial which is different from that of the gate electrode or the gatewiring may also be called either a gate electrode or a gate wiring.

Note that a gate terminal corresponds to part of a portion (a region, aconductive film, a wiring, or the like) of a gate electrode or a portion(a region, a conductive film, a wiring, or the like) which iselectrically connected to the gate electrode.

Note that when a wiring is called a gate wiring, a gate line, a gatesignal line, a scan line, a scan signal line, there is the case in whicha gate of a transistor is not connected to a wiring. In this case, thegate wiring, the gate line, the gate signal line, the scan line, or thescan signal line corresponds to a wiring formed in the same layer as thegate of the transistor, a wiring formed of the same material of the gateof the transistor, or a wiring formed at the same time as the gate ofthe transistor in some cases. As examples, a wiring for storagecapacitance, a power supply line, a reference potential supply line, andthe like can be given.

Note also that a source corresponds to all or part of a source region, asource electrode, and a source wiring (also referred to as a sourceline, a source signal line, a data line, a data signal line, or thelike). A source region corresponds to a semiconductor region including alarge amount of p-type impurities (e.g., boron or gallium) or n-typeimpurities (e.g., phosphorus or arsenic). Therefore, a region includinga small amount of p-type impurities or n-type impurities, namely, an LDD(lightly doped drain) region is not included in the source region. Asource electrode is part of a conductive layer fanned of a materialdifferent from that of a source region, and electrically connected tothe source region. However, there is the case where a source electrodeand a source region are collectively called a source electrode. A sourcewiring is a wiring for connecting a source electrode of each transistorto each other, a wiring for connecting a source electrode of each pixelto each other, or a wiring for connecting a source electrode to anotherwiring.

However, there is a portion (a region, a conductive film, a wiring, orthe like) functioning as both a source electrode and a source wiring.Such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring. That is, thereis a region where a source electrode and a source wiring cannot beclearly distinguished from each other. For example, in the case where asource region overlaps with part of an extended source wiring, theoverlapped portion (region, conductive film, wiring, or the like)functions as both a source wiring and a source electrode. Accordingly,such a portion (a region, a conductive film, a wiring, or the like) maybe called either a source electrode or a source wiring.

In addition, a portion (a region, a conductive film, a wiring, or thelike) which is formed of the same material as a source electrode, formsthe same island as the source electrode, and is connected to the sourceelectrode, or a portion (a region, a conductive film, a wiring, or thelike) which connects a source electrode and another source electrode mayalso be called a source electrode. Further, a portion which overlapswith a source region may be called a source electrode. Similarly, aportion (a region, a conductive film, a wiring, or the like) which isformed of the same material as a source wiring, forms the same island asthe source wiring, and is connected to the source wiring may also becalled a source wiring. In a strict sense, such a portion (a region, aconductive film, a wiring, or the like) does not have a function ofconnecting the source electrode to another source electrode in somecases. However, there is a portion (a region, a conductive film, awiring, or the like) which is formed of the same material as a sourceelectrode or a source wiring, forms the same island as the sourceelectrode or the source wiring, and is connected to the source electrodeor the source wiring because of conditions in a manufacturing step.Thus, such a portion (a region, a conductive film, a wiring, or thelike) may also be called either a source electrode or a source wiring.

In addition, for example, part of a conductive film which connects asource electrode and a source wiring and is formed of a material whichis different from that of the source electrode or the source wiring maybe called either a source electrode or a source wiring.

Note that a source terminal corresponds to part of a source region, asource electrode, or a portion (a region, a conductive film, a wiring,or the like) which is electrically connected to the source electrode.

Note that when a wiring is called a source wiring, a source line, asource signal line, a data line, a data signal line, there is the casein which a source (a drain) of a transistor is not connected to awiring. In this case, the source wiring, the source line, the sourcesignal line, the data line, or the data signal line corresponds to awiring formed in the same layer as the source (the drain) of thetransistor, a wiring formed of the same material of the source (thedrain) of the transistor, or a wiring formed at the same time as thesource (the drain) of the transistor in some cases. As examples, awiring for storage capacitance, a power supply line, a referencepotential supply line, and the like can be given.

Note also that the same can be said for a drain.

Note also that a semiconductor device corresponds to a device having acircuit including a semiconductor element (e.g., a transistor, a diode,or thyristor). The semiconductor device may also include all devicesthat can function by utilizing semiconductor characteristics.

Note also that a display element corresponds to an optical modulationelement, a liquid crystal element, a light-emitting element, an ELelement (an organic EL element, an inorganic EL element, or an ELelement including organic and inorganic materials), an electron emitter,an electrophoresis element, a discharging element, a light-reflectiveelement, a light diffraction element, a digital micro device (DMD), orthe like. Note that the present invention is not limited to this.

In addition, a display device corresponds to a device having a displayelement. Note that the display device may also corresponds to a displaypanel itself where a plurality of pixels including display elements areformed over the same substrate as a peripheral driver circuit fordriving the pixels. In addition, the display device may also include aperipheral driver circuit provided over a substrate by wire bonding orbump bonding, namely, an IC chip connected by chip on glass (COG) or anIC chip connected by TAB or the like. Further, the display device mayalso include a flexible printed circuit (FPC) to which an IC chip, aresistor, a capacitor, an inductor, a transistor, or the like isattached. Note also that the display device includes a printed wiringboard (PWB) which is connected through a flexible printed circuit (FPC)and to which an IC chip, a resistor, a capacitor, an inductor, atransistor, or the like is attached. The display device may also includean optical sheet such as a polarizing plate or a retardation plate. Thedisplay device may also include a lighting device, a housing, an audioinput and output device, a light sensor, or the like. Here, a lightingdevice such as a backlight unit may include a light guide plate, a prismsheet, a diffusion sheet, a reflective sheet, a light source (e.g., anLED or a cold cathode fluorescent lamp), a cooling device (e.g., a watercooling device or an air cooling device), or the like.

Moreover, a lighting device corresponds to a device having a backlightunit, a light guide plate, a prism sheet, a diffusion sheet, areflective sheet, or a light source (e.g., an LED, a cold cathodefluorescent lamp, or a hot cathode fluorescent lamp), a cooling device,or the like.

In addition, a light-emitting device corresponds to a device having alight-emitting element and the like. In the case of including alight-emitting element as a display element, the light-emitting deviceis one of specific examples of a display device.

Note that a reflective device corresponds to a device having alight-reflective element, a light diffraction element, light-reflectiveelectrode, or the like.

A liquid crystal display device corresponds to a display deviceincluding a liquid crystal element. Liquid crystal display devicesinclude a direct-view liquid crystal display, a projection liquidcrystal display, a transmissive liquid crystal display, a reflectiveliquid crystal display, a semi-transmissive liquid crystal display, andthe like.

Note also that a driving device corresponds to a device having asemiconductor element, an electric circuit, or an electronic circuit.For example, a transistor which controls input of a signal from a sourcesignal line to a pixel (also referred to as a selection transistor, aswitching transistor, or the like), a transistor which supplies voltageor current to a pixel electrode, a transistor which supplies voltage orcurrent to a light-emitting element, and the like are examples of thedriving device. A circuit which supplies a signal to a gate signal line(also referred to as a gate driver, a gate line driver circuit, or thelike), a circuit which supplies a signal to a source signal line (alsoreferred to as a source driver, a source line driver circuit, or thelike) are also examples of the driving device.

Note also that a display device, a semiconductor device, a lightingdevice, a cooling device, a light-emitting device, a reflective device,a driving device, and the like overlap with each other in some cases.For example, a display device includes a semiconductor device and alight-emitting device in some cases. Alternatively, a semiconductordevice includes a display device and a driving device in some cases.

In this document (the specification, the claim, the drawing, and thelike), when it is explicitly described that “B is formed on A” or “B isformed over A”, it does not necessarily mean that B is formed in directcontact with A. The description includes the case where A and B are notin direct contact with each other, i.e., the case where another objectis interposed between A and B. Here, each of A and B corresponds to anobject (e.g., a device, an element, a circuit, a wiring, g, anelectrode, a terminal, a conductive film, or a layer).

Accordingly, for example, when it is explicitly described that a layer Bis formed on (or over) a layer A, it includes both the case where thelayer B is formed in direct contact with the layer A, and the case whereanother layer (e.g., a layer C or a layer D) is formed in direct contactwith the layer A and the layer B is formed in direct contact with thelayer C or D. Note that another layer (e.g., a layer C or a layer D) maybe a single layer or a plurality of layers.

Similarly, when it is explicitly described that B is formed above A, itdoes not necessarily mean that B is formed in direct contact with A, andanother object may be interposed therebetween. Accordingly, for example,when it is explicitly described that a layer B is formed above a layerA, it includes both the case where the layer B is formed in directcontact with the layer A, and the case where another layer (e.g., alayer C or a layer D) is formed in direct contact with the layer A andthe layer B is formed in direct contact with the layer C or D. Note thatanother layer (e.g., a layer C or a layer D) may be a single layer or aplurality of layers.

Note that when it is explicitly described that B is formed in directcontact with A, it includes not the case where another object isinterposed between A and B but the case where B is formed in directcontact with A.

Note that the same can be said when it is explicitly described that B isformed below or under A.

By using the present invention, a hold-type display device without aproblem of motion blur and a driving method thereof can be provided. Inaddition, by using the present invention, a display device with lowpower consumption and a driving method thereof can be provided. Further,by using the present invention, a display device with improved qualityfor still images and moving images and a driving method thereof can beprovided. Furthermore, by using the present invention, a display devicewith a wider viewing angle and a driving method thereof can be provided.Moreover, by using the present invention, a display device with improvedresponse speed of a liquid crystal and a driving method thereof can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are diagrams each illustrating definitions of words andsigns in accordance with the present invention;

FIGS. 2A to 2C are diagrams each illustrating definitions of words andsigns in accordance with the present invention;

FIGS. 3A and 3B are diagrams each illustrating an example of a conditionof integrated luminance with respect to control parameters in accordancewith the present invention;

FIGS. 4A, 4C, 4E, and 4G are diagrams each illustrating an example of acondition of a lighting ratio with respect to control parameters inaccordance with the present invention, and FIGS. 4B, 4D, 4F, and 4H arediagrams each illustrating an example of a condition of averageluminance with respect to control parameters in accordance with thepresent invention;

FIGS. 5A to 5C are diagrams each illustrating an example of conditionsof a lighting ratio and average luminance with respect to controlparameters in accordance with the present invention;

FIGS. 6A to 6P are diagrams each illustrating an example of a conditionof a lighting ratio with respect to control parameters in accordancewith the present invention;

FIGS. 7A to 7E are diagrams each illustrating an example of a conditionof a lighting ratio with respect to control parameters in accordancewith the present invention;

FIGS. 8A to 8G are diagrams each illustrating an example of a conditionof a lighting ratio with respect to control parameters in accordancewith the present invention;

FIGS. 9A to 9F are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention, and FIGS. 9G and 9H are diagrams each illustrating an exampleof a pixel circuit of a semiconductor device in accordance with thepresent invention;

FIGS. 10A and 10B are diagrams each illustrating examples of a timingchart and a display condition of a semiconductor device in accordancewith the present invention;

FIGS. 11A to 11J are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 12A and 12B are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 13A to 13C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 14A, 14B, 14E, and 14F are diagrams each showing an example ofluminance with respect to a gray scale of a semiconductor device inaccordance with the present invention, and FIGS. 14C, 14D, 14G, and 14Hare diagrams each showing an example of the number of data with respectto a gray scale of a semiconductor device in accordance with the presentinvention;

FIGS. 15A and 15B are diagrams each showing an example of luminance withrespect to a gray scale of a semiconductor device in accordance with thepresent invention, and FIGS. 15C and 15D are diagrams each showing anexample of the number of data with respect to a gray scale of asemiconductor device in accordance with the present invention;

FIGS. 16A to 16D are diagrams each showing an example of luminance withrespect to a gray scale of a semiconductor device in accordance with thepresent invention;

FIGS. 17A to 17L are views each illustrating an example of controlparameters in accordance with the present invention;

FIGS. 18A to 18I are views each illustrating an example of controlparameters in accordance with the present invention, and FIGS. 18J to18L are diagrams in which histograms of images shown in FIGS. 18A to 18Iare compared with each other;

FIGS. 19A to 19C are views each illustrating an example of controlparameters in accordance with the present invention;

FIGS. 20A to 20C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 21A to 21D are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention, and FIGS. 21E and 21F are diagrams each illustrating anexample of a driver circuit of a semiconductor device in accordance withthe present invention;

FIGS. 22A to 22D are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 23A to 23D are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 24A and 24B are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 25A to 25C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 26A to 26C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 27A to 27C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 28A to 28C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 29A to 29C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 30A to 30C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 31A to 31C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 32A to 32C are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIG. 33A is a diagram illustrating an example of a circuit structure ofa semiconductor device in accordance with the present invention, andFIG. 33B is a diagram of an example of a timing chart of a semiconductordevice in accordance with the present invention;

FIGS. 34A to 34BII are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 35A to 35BII are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 36A to 36BII are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIG. 37 is a view illustrating an example of a cross-sectional view of asemiconductor device in accordance with the present invention;

FIGS. 38A and 38B are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIGS. 39A and 39B are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIG. 40 is a view illustrating an example of a pixel layout of asemiconductor device in accordance with the present invention;

FIGS. 41A and 41B are views each illustrating an example of a pixellayout of a semiconductor device in accordance with the presentinvention;

FIGS. 42A and 42B are views each illustrating an example of a pixellayout of a semiconductor device in accordance with the presentinvention;

FIG. 43 is a view illustrating an example of a cross-sectional view of asemiconductor device in accordance with the present invention;

FIGS. 44A to 44D are views each illustrating an example of a peripheralcomponent of a semiconductor device in accordance with the presentinvention;

FIG. 45 is a view illustrating an example of a peripheral component of asemiconductor device in accordance with the present invention;

FIGS. 46A to 46C are diagrams each showing an example of a circuitstructure of a panel of a semiconductor device in accordance with thepresent invention;

FIGS. 47A and 47B are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIGS. 48A to 48C are diagrams each illustrating an example of a drivingmethod of a semiconductor device in accordance with the presentinvention;

FIGS. 49A and 49B are diagrams each illustrating an example of a circuitstructure of a semiconductor device in accordance with the presentinvention;

FIGS. 50A to 50C are diagrams each illustrating an example of aperipheral component of a semiconductor device in accordance with thepresent invention;

FIGS. 51A and 51B are diagrams each illustrating an example of a circuitstructure of a semiconductor device in accordance with the presentinvention;

FIG. 52 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIG. 53 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIGS. 54A and 54B are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIGS. 55A to 55D are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIGS. 56A to 56D are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIGS. 57A to 57D are views each illustrating an example of across-sectional view of a semiconductor device in accordance with thepresent invention;

FIG. 58 is a view illustrating an example of a top plan view of asemiconductor device in accordance with the present invention;

FIGS. 59A to 59D are views each illustrating an example of a top planview of a semiconductor device in accordance with the present invention;

FIGS. 60A to 60D are views each illustrating an example of a top planview of a semiconductor device in accordance with the present invention;

FIG. 61A is a view illustrating an example of a pixel layout of asemiconductor device in accordance with the present invention, and FIG.61B is a view illustrating an example of a cross-sectional view thereof;

FIG. 62A is a view illustrating an example of a pixel layout of asemiconductor device in accordance with the present invention, and FIG.62B is a view illustrating an example of a cross-sectional view thereof;

FIG. 63A is a view illustrating an example of a pixel layout of asemiconductor device in accordance with the present invention, and FIG.63B is a view illustrating an example of a cross-sectional view thereof;

FIGS. 64A and 64B are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIGS. 65A and 65B are diagrams each illustrating an example of a timingchart of a semiconductor device in accordance with the presentinvention;

FIG. 66 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIG. 67 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIG. 68 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIG. 69 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIG. 70 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIGS. 71A to 71G are cross-sectional views each illustrating amanufacturing process of a semiconductor device in accordance with thepresent invention;

FIG. 72 is a view illustrating an example of a cross-sectional view of asemiconductor device in accordance with the present invention;

FIG. 73 is a view illustrating an example of a cross-sectional view of asemiconductor device in accordance with the present invention;

FIG. 74 is a view illustrating an example of a cross-sectional view of asemiconductor device in accordance with the present invention;

FIG. 75 is a view illustrating an example of a cross-sectional view of asemiconductor device in accordance with the present invention;

FIGS. 76A to 76C are cross-sectional views each illustrating an exampleof a display element of a semiconductor device in accordance with thepresent invention;

FIGS. 77A to 77C are cross-sectional views each illustrating an exampleof a display element of a semiconductor device in accordance with thepresent invention;

FIGS. 78A and 78B are views each illustrating an example of a structureof a semiconductor device in accordance with the present invention;

FIG. 79 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIG. 80 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIG. 81 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIGS. 82A to 82C are views each illustrating an example of a structureof a semiconductor device in accordance with the present invention;

FIG. 83 is a diagram illustrating an example of a circuit structure of asemiconductor device in accordance with the present invention;

FIG. 84 is a diagram illustrating an example of a timing chart of asemiconductor device in accordance with the present invention;

FIG. 85 is a diagram illustrating an example of a timing chart of asemiconductor device in accordance with the present invention;

FIGS. 86A and 86B are views each illustrating an example of a drivingmethod of a semiconductor device in accordance with the presentinvention;

FIGS. 87A to 87E are cross-sectional views each illustrating an exampleof a display element of a semiconductor device in accordance with thepresent invention;

FIG. 88 is a view illustrating an example of a manufacturing apparatusof a semiconductor device in accordance with the present invention;

FIG. 89 is a view illustrating an example of a manufacturing device of asemiconductor device in accordance with the present invention;

FIG. 90 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIG. 91 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIGS. 92A and 92B are views each illustrating an example of a structureof a semiconductor device in accordance with the present invention;

FIGS. 93A and 93B are views each illustrating an example of a structureof a semiconductor device in accordance with the present invention;

FIG. 94 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIG. 95 is a view illustrating an example of a structure of asemiconductor device in accordance with the present invention;

FIGS. 96A to 96H are views each illustrating an electronic device usinga semiconductor device in accordance with the present invention;

FIG. 97 is a view illustrating an electronic device using asemiconductor device in accordance with the present invention;

FIG. 98 is a view illustrating an electronic device using asemiconductor device in accordance with the present invention;

FIG. 99 is a view illustrating an electronic device using asemiconductor device in accordance with the present invention;

FIG. 100 is a view illustrating an electronic device using asemiconductor device in accordance with the present invention;

FIGS. 101A and 101B are views each illustrating an electronic deviceusing a semiconductor device in accordance with the present invention;and

FIGS. 102A and 102B are views each illustrating an electronic deviceusing a semiconductor device in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described by way ofembodiment modes with reference to the drawings. However, the presentinvention can be implemented in various different ways and it will beeasily understood by those skilled in the art that various changes andmodifications are possible. Unless such changes and modifications departfrom the spirit and the scope of the present invention, they should beconstrued as being included therein. Therefore, the present inventionshould not be construed as being limited to the description of theembodiment modes.

Embodiment Mode 1

In this embodiment mode, words relating to a driving method of a displaydevice, such as instantaneous luminance, integrated luminance, alighting ratio, and average luminance used in this document (thespecification, the claim, the drawing, and the like), and control modesthereof are described.

First, meanings of words and signs used in this document are described.First, words about time and signs thereof, i.e., t, F, τ_(a), τ_(b), andR are described. The sign t expresses time. The sign F expresses oneframe period and the length thereof. One frame period F is divided intoa plurality of subframe periods, and each of the subframe periods areclassified into an image display period or a blanking interval. Here,the image display period is a period during which original luminance ofan image is mainly displayed. The blanking interval is a period duringwhich an image displayed in the image display period can be reset byhuman eyes. Note that the subframe period may be a period other than theimage display period and the blanking interval. The sign τ_(a) expressesthe image display period and the length thereof. The sign τ_(b)expresses the blanking interval and the length thereof. Note also thatperiods other than the image display period τ_(a) in the one frameperiod F each correspond to the blanking interval Σ_(b). The sign Rexpresses a lighting ratio. Here, the lighting ratio is a value obtainedby dividing the image display period τ_(a) by the one frame period F.That is, the lighting ratio R is a ratio of the image display periodτ_(a) in the one frame period F.

Next, words relating to luminance and signs thereof, i.e., I (t), L, andB are described. The sign I (t) shows instantaneous luminance. Here,instantaneous luminance is instantaneous emission intensity of a pixel.The sign L expresses integrated luminance. Here, the integratedluminance is a value obtained by integrating the instantaneous luminanceI (t) by time in a range of the one frame period F. The sign B expressesaverage luminance. Here, the average luminance is a value obtained bydividing the integrated luminance L by the image display period τ_(a).That is, the average luminance B expresses luminance of a pixel when itis assumed that the luminance of the pixel is constant in the imagedisplay period τ_(a).

When the above-described relations are represented by formulas, therelations can be represented by Formula 1 to Formula 4.

$\begin{matrix}{R = \frac{\tau_{a}}{F}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack \\{\tau_{b} = {F - \tau_{a}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack \\{L = {\int_{F}{{I(t)}{t}}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack \\{B = \frac{L}{\tau_{a\;}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

Hereinafter, when the above-described signs are used without notice inthis document, meanings thereof may be considered that they follow theabove-described definitions.

Note that an actual relationship between the words which are defined inthis document and display conditions of a display device are described.As for the lighting ratio R, driving becomes closer to hold-type drivingas R increases, and driving becomes closer to impulsive driving as Rdecreases. As for the integrated luminance L, luminance perceived byhuman eyes increases as L increases, and luminance perceived by humaneyes decreases as L decreases. As for the instantaneous luminance I (t),either increase or decrease in the integrated luminance I (t) does nothave a direct relation that luminance perceived by human eyes increasesor decreased. This is because human eyes cannot perceive fluctuation inlight, brightness of which fluctuates with high frequency and perceivesthe light as constant light. At this time, brightness perceived by humaneyes is not fluctuation in brightness itself but light having afrequency of a value obtained by integrating brightness by time in acertain range. In addition, limit frequency in which human eyes perceivefluctuation in brightness is approximately 50 Hz to 60 Hz. Thisfrequency is almost the same value as a frame rate of a general displaydevice. Further, the average luminance B is an amount used forconvenience to describe control modes in this document; however, theaverage luminance B is similar to the integrated luminance L in thatluminance perceived by human eyes increases as B increases, andluminance perceived by human eyes decreases as B decreases.

Next, the words in this document are described in detail with referenceto FIGS. 1A to 1C. Here, one of pixels included in a display device isfocused on, and instantaneous luminance I (t) of the pixel isschematically shown in FIGS. 1A to 1C. A horizontal axis represents timet and a vertical axis represents luminance.

FIG. 1A is an example of the case in which one frame period is dividedinto two subframe periods, a first subframe period corresponds to animage display period τ_(a), and a second subframe period corresponds toa blanking interval Σ_(b). In FIG. 1A, each of the image display periodτ_(a) and the blanking interval τ_(b) is half of one frame period F(τ_(a)=τ_(b)=F/2 is satisfied). In addition, instantaneous luminance I(t) in the image display period τ_(a) is constant and a value thereof isa. Instantaneous luminance I (t) in the blanking interval τ_(b) isconstant and a value thereof is 0. At this time, as for lighting ratioR, R=(τ_(a)/F)=1/2 is satisfied. As for integrated luminance L,L=a×(F/2)=(aF/2) is satisfied. Therefore, as for average luminance B,B=(L/τ_(a))=a is satisfied.

FIG. 1B is a diagram showing the case in which there are a plurality ofimage display periods τ_(a) and a plurality of blanking intervals τ_(b).In this manner, the image display period τ_(a) and the blanking intervalτ_(b) can be divided into a plurality of sub-image display periods andsub-blanking intervals. That is, when there are n (n is a positiveinteger) pieces of sub-image display periods in one frame period, thesub-image display periods are denoted by τ_(a1), τ_(a2), τ_(a3), . . . ,and τ_(an) and the image display period Σ_(a) is the sum thereof.Similarly, when there are n (n is a positive integer) pieces ofsub-blanking intervals in one frame period, the sub-blanking intervalsare denoted by τ_(b1), τ_(b2), τ_(b3), . . . , and τ_(bn) and theblanking interval τ_(b) is the sum thereof. That is, when there are npieces of sub-image display periods and sub-blanking intervals in oneframe period, the image display period τ_(a) and the blanking intervalτ_(b) can be represented by Formula 5 and Formula 6 when j and k arepositive integers.

$\begin{matrix}{\tau_{a} = {\sum\limits_{j = 1}^{n}\tau_{a\; j}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack \\{\tau_{b} = {\sum\limits_{k = 1}^{n}\tau_{b\; k}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack\end{matrix}$

In FIG. 1B, as for the sub-image display period, τ_(a1)=τ_(a2)=F/4 issatisfied, and as for the sub-blanking interval, τ_(b1)=τ_(b2)=F/4 issatisfied. Therefore, as for the image display period τ_(a) and theblanking interval τ_(b), τ_(a)=τ_(b)=F/2 is satisfied. Instantaneousluminance I (t) in the image display period τ_(a) is constant and avalue thereof is a. Instantaneous luminance I (t) in the blankinginterval τ_(b) is constant and a value thereof is 0. At this time, asfor lighting ratio R, R=(τ_(a)/F)=1/2 is satisfied. As for integratedluminance L, L=a×(F/4)+a(F/4)=(aF/2) is satisfied. Therefore, as foraverage luminance B, B=(L/τ_(a))=a is satisfied.

FIG. 1C shows the case in which there are a plurality of image displayperiods τ_(a) and a plurality of blanking intervals τ_(b), andinstantaneous luminance is different in each of the sub-image displayperiod. In FIG. 1C, as for the sub-image display period,τ_(a1)=τ_(a2)=F/4 is satisfied, and as for the sub-blanking interval,τ_(b1)=τ_(b2)=F/4 is satisfied. Therefore, as for the image displayperiod τ_(a) and the blanking interval τ_(b), τ_(a)=τ_(b)=F/2 issatisfied. Instantaneous luminance I (t) in the sub-image display periodτ_(a1) is constant and a value thereof is a/2. Instantaneous luminance I(t) in the sub-image display period τ_(a2) is constant and a valuethereof is 3a/2. Instantaneous luminance I (t) in the blanking intervalτ_(b) is constant and a value thereof is 0. At this time, as forlighting ratio R, R=(τ_(a)/F)=1/2 is satisfied. As for integratedluminance L, L=(a/2)(F/4)+(3a/2)(F/4)=(aF/2) is satisfied. Therefore, asfor average luminance B, B=(L/τ_(a))=a is satisfied.

The lighting ratio R, the integrated luminance L, and the averageluminance B, which are values used in this document, are the samebetween the examples shown in FIGS. 1A to 1C, although the instantaneousluminance I (t) is a different condition in each of FIGS. 1A to 1C. Thatis, this embodiment mode mainly describes how the lighting ratio R, theintegrated luminance L, and the average luminance B are controlled;however, here, it is emphasized that even when the lighting ratio R, theintegrated luminance L, and the average luminance B are the same,instantaneous luminance I (t) with respect to them can be varied.

FIGS. 2A to 2C are diagrams each schematically showing instantaneousluminance I (t) of the case of a display device using an element havingcharacteristics which change slowly in response to a signal (e.g., aliquid crystal element). Even when a signal controlling the element isinput similarly to FIGS. 1A to 1C, instantaneous luminance I (t) of thecase of the display device using the element having characteristicswhich change slowly in response to the signal with delay.

However, in accordance with the definitions of this document, thelighting ratio R, the integrated luminance L, and the average luminanceB can be calculated without a problem even in such a case.

The image display period τ_(a) and the blanking interval τ_(b) may bedetermined based on a period during which a signal controlling luminanceis input or may be determined based on the instantaneous luminance I(t). When the image display period τ_(a) and the blanking interval τ_(b)are determined based on the period during which the signal controllingthe luminance is input, the time when the signal is updated is aboundary between the periods. When the image display period τ_(a) andthe blanking interval τ_(b) are determined based on the instantaneousluminance I (t), the time at which change in the instantaneous luminanceI (t) is drastic is a boundary between the periods. More specifically,the time t at which a primary function is discontinuous is a boundarybetween the periods. For example, in the case of FIG. 2A, the imagedisplay period τ_(a) and the blanking interval τ_(b) are determined bysetting time t₁ at which increase in the instantaneous luminance I (t)begins to decrease as a boundary between the periods. In the case ofFIG. 2B, the image display period τ_(a) and the blanking interval τ_(b)are determined by setting time t₁ at which increase in the instantaneousluminance I (t) begins to decrease as a first boundary of the periods,setting time t₂ at which decrease in the instantaneous luminance I (t)begins to increase as a second boundary of the periods, and setting timet₃ at which decrease in the instantaneous luminance I (t) begins toincrease again as a third boundary of the periods. FIG. 2C is similar tothe case of FIG. 2B. When the image display period τ_(a) and theblanking interval τ_(b) are determined, the lighting ratio R can becalculated by Formula 1.

The integrated luminance L can be calculated by Formula 3 fromconditions of the instantaneous luminance I (t). In this manner, theintegrated luminance L can be calculated by Formula 3 even when theinstantaneous luminance I (t) is a given function.

The average luminance B can be calculated by Formula 4 from the imagedisplay period τ_(a) and the instantaneous luminance I (t) calculated bythe above-described method.

The blanking interval τ_(b) is provided in one frame period so thatquality of a moving image displayed by a display device is improved.Therefore, as far as quality of a moving image displayed by a displaydevice is improved in a period, the period can be considered as theblanking interval τ_(b) regardless of luminance of a pixel in theperiod.

Luminance of a pixel in the blanking interval τ_(b) is preferablyluminance in which luminance of the pixel in the image display periodτ_(a) can be reset by human eyes. Therefore, the luminance of the pixelin the blanking interval τ_(b) is preferably lower than the luminance ofthe pixel in the image display period τ_(a). More preferably, theluminance of the pixel in the blanking interval τ_(b) is the lowestluminance within display capability of the display device.

Next, control modes of the values used in this document are described.In this embodiment mode, change in the integrated luminance L, thelighting ratio R, and the average luminance B by a control parameter Pare particularly described.

Although various parameters can be given as the control parameter P,details of the control parameter P is not described in this embodimentmode. Details of the control parameter P is described in anotherembodiment mode, and this embodiment mode describes how the integratedluminance L, the lighting ratio R, and the average luminance B arechanged simply in accordance with increase and decrease in the controlparameter P.

Note that when change in the integrated luminance L, the lighting ratioR, and the average luminance B with respect to change in the controlparameter P is described, it is assumed that luminance of pixelsperceived by human eyes is the same.

First, change in the integrated luminance L with respect to change inthe control parameter P is described with reference to FIGS. 3A and 3B.Change in the integrated luminance L with respect to change in thecontrol parameter P can be described in detail by a graph in which ahorizontal axis represents the control parameter P and a vertical axisrepresents the integrated luminance L, as in FIGS. 3A and 3B.

The integrated luminance L is preferably almost constant with respect toincrease in the control parameter P. This is because change in theintegrated luminance L corresponds to change in luminance, which isperceived by human eyes, and drastic change in the integrated luminanceL cannot be allowed under the assumption that luminance of the pixels,which is perceived by human eyes is the same. This condition can beunderstood with reference to FIG. 3A. In a graph shown in FIG. 3A, L=L₀when P=0, and L=L₀ is always satisfied even when P becomes larger than0.

Here, when the integrated luminance L is considered as a functionparticularly with respect to the control parameter P, the integratedluminance L is referred to as integrated luminance L (P). That is, whenthe graph shown in FIG. 3A is represented by a formula, L(P)=L(0)=L₀.

Note that actually, it is not necessary that L(P)=L₀ be strictlysatisfied, and there may be a certain range. This condition can also beunderstood with reference to FIG. 3A. In the graph shown in FIG. 3A, afluctuation range of the integrated luminance L, which can be allowed,is shown by two broken lines. A formula of the broken like isL(P)=L₀±(L₀/20). That is, it is only necessary that the integratedluminance L be within a range having a central value of L₀ and a widthof L₀/10 with respect to change in the control parameter P. Whenfluctuation of the integrated luminance L is in the range, fluctuationof the integrated luminance L can be allowed. This is because whenfluctuation of the integrated luminance L is small, it is not perceivedas fluctuation in luminance, and fluctuation of the integrated luminanceL is extremely small even when fluctuation of the integrated luminance Lis perceived as fluctuation in luminance.

In addition, the integrated luminance L may be increased slowly withrespect to increase in the control parameter P. This is because whenchange in the integrated luminance L is small, this change is allowed,and display can be emphasized in accordance with increase in the controlparameter P when the integrated luminance increases slowly with respectto increase in the control parameter P. This condition can be understoodwith reference to FIG. 3B. In a graph shown in FIG. 3B, L=L₀ when P=0,and L=L₀ is gradually increased as P increases from 0.

Here, when the graph shown in FIG. 3B is represented by a formula,L(P)=αP+L₀. α is a proportional constant and a positive number which islarger than 0. In addition, the proportional constant α is preferablysmaller than 1. This is because change in the integrated luminance L issmall when the proportional constant α is small, and change in theintegrated luminance L can be allowed.

Note that it is not necessary that L(P)=αP+L₀ be strictly satisfied, andthere may be a certain range. This condition can also be understood withreference to FIG. 3B. In the graph shown in FIG. 3B, a fluctuation rangeof the integrated luminance L, which can be allowed, is shown by twobroken lines. A formula of the broken like is L (P)=αP+L₀±(L₀/20). Thatis, it is only necessary that the integrated luminance L be within arange having a central value of αP+L₀ and a width of L₀/10 with respectto change in the control parameter P. When fluctuation of the integratedluminance L is in the range, fluctuation of the integrated luminance Lcan be allowed. This is because when fluctuation of the integratedluminance L is small, it is not perceived as fluctuation in luminance,and fluctuation of the integrated luminance L is extremely small evenwhen fluctuation of the integrated luminance L is perceived asfluctuation in luminance.

Next, change in the lighting ratio R and the average luminance B withrespect to the control parameter P is described with reference to FIGS.4A to 4H. Change in the lighting ratio R and the average luminance Bwith respect to the control parameter P can be described in detail by agraph in which a horizontal axis represents the control parameter P anda vertical axis represents the lighting ratio R or the average luminanceB. FIGS. 4A, 4C, 4E, and 4G are graphs each showing change in lightingratio R with respect to the control parameter P. FIGS. 4B, 4D, 4F, and4H are graphs each showing change in the average luminance B withrespect to the control parameter P.

FIG. 4A shows the case where the lighting ratio R is almost constantwith respect to increase in the control parameter P. Change in thelighting ratio R corresponds to how a ratio of the image display periodτ_(a) in the one frame period F is changed. This is because on acondition that the integrated luminance L is constant with respect tothe control parameter P, the lighting ratio R is almost constant withrespect to the control parameter P when the average luminance B isalmost constant with respect to the control parameter P. This conditioncan be understood with reference to the following description and FIG.4A.

The fluctuation range of the integrated luminance L with respect to thecontrol parameter P, which can be allowed, is extremely small, which hasbeen already described. Future discussions will be proceeded on acondition that the integrated luminance L is almost constant withrespect to the control parameter P.

When Formula 1 and Formula 4 are transformed to be organized, Formula 7can be obtained.

$\begin{matrix}{{B\; R} = \frac{L}{F}} & \left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack\end{matrix}$

Here, the integrated luminance L is almost constant with respect to thecontrol parameter P. In addition, when the one frame period F is alsoalmost constant with respect to the control parameter P, the right sideof Formula 7 is almost constant with respect to the control parameter P.Therefore, a product of the lighting ratio R and the average luminance Bis almost constant with respect to the control parameter P.

Thus, from the fact that the product of the lighting ratio R and theaverage luminance B is almost constant with respect to the controlparameter P, a conclusion that the lighting ratio R is almost constantwith respect to increase in the control parameter P when the averageluminance B is almost constant with respect to the control parameter Pcan be obtained.

Change in the lighting ratio R with respect to increase in the controlparameter P is described with reference to FIG. 4A. When the lightingratio R is considered as a function particularly with respect to thecontrol parameter P, the lighting ratio R is referred to as lightingratio R (P). In addition, R=R₀ when P=0. That is, when the graph shownin FIG. 4A is represented by a formula, R(P)=R(0)=R₀.

Note that actually, it is only necessary that R (P) be in a range ofapproximately R₀/10 setting R_(o) as a certain value even when R(P)=R₀is not strictly satisfied.

Change in the average luminance B with respect to the control parameterP is described with reference to FIG. 4B. When the average luminance Bis considered as a function particularly with respect to the controlparameter P, the average luminance B is referred to as average luminanceB (P). In addition, B=B₀ when P=0. That is, when the graph shown in FIG.4B is represented by a formula, B(P)=B(0)=B₀.

Note that actually, it is only necessary that B (P) be in a range ofapproximately B₀/10 setting B_(o) as a certain value even when B(P)=B₀is not strictly satisfied.

Next, the lighting ratio R can be simply decreased with respect toincrease in the control parameter P. This is because when on a conditionthat the product of the lighting ratio R and the average luminance B isalmost constant with respect to the control parameter P, the lightingratio R monotonously decreases with respect to the control parameter Pwhen the average luminance B monotonously increases with respect to thecontrol parameter P. This condition can be understood with reference toFIGS. 4C to 4H.

In each of graphs shown in FIGS. 4C, 4E, and 4G, the lighting ratio R issimply decreased with respect to the control parameter P. As in thegraph shown in FIG. 4C, the lighting ratio R may decrease linearly withrespect to the control parameter P. Alternatively, as in the graph shownin FIG. 4E, the lighting ratio R may decrease as shown by an upwardcurving line with respect to the control parameter P. Furtheralternatively, as in the graph shown in FIG. 4G, the lighting ratio Rmay decrease as shown by a downward curving line with respect to thecontrol parameter P.

When the lighting ratio R decreases linearly with respect to the controlparameter P, the average luminance B increases linearly with respect tothe control parameter P as in a graph shown in FIG. 4D.

When the lighting ratio R decreases as shown by an upward curving linewith respect to with respect to the control parameter P, the averageluminance B increases as shown by a downward curving line with respectto the control parameter P as in a graph shown in FIG. 4F.

When the lighting ratio R decreases as shown by a downward curving linewith respect to with respect to the control parameter P, the averageluminance B increases as shown by an upward curving line with respect tothe control parameter P as in a graph shown in FIG. 4H.

When a value of the lighting ratio R is constant, it is not necessarythat the control modes be changed precisely with respect to change inthe control parameter P. Accordingly, since algorithm which determines adisplay method and a peripheral circuit which makes many control modesto be selected are not needed, manufacturing cost of the display devicecan be reduced. In addition, since the size of a circuit and frequencyof operation can be reduced, power consumption can be reduced.

When a value of the lighting ratio R decreases linearly, the controlmodes can be changed precisely with respect to change in the controlparameter P. Accordingly, by using algorithm which determines a displaymethod and a peripheral circuit which makes many control modes to beselected, suitable control modes in accordance with the controlparameter P can be realized. Therefore, high-quality display with littlemotion blur and little flicker can be obtained.

When a value of the lighting ratio R decreases as shown by an upwardcurving line, the control modes can be changed precisely with respect tochange in the control parameter P. In addition, the amount of change inthe lighting ratio R can be increased as the control parameter P becomeslarger. Accordingly, by using algorithm which determines a displaymethod and a peripheral circuit which makes many control modes to beselected, more suitable control modes in accordance with the controlparameter P can be realized. Therefore, higher-quality display withlittle motion blur and little flicker can be obtained.

When a value of the lighting ratio R decreases as shown by a downwardcurving line, the control modes can be changed finely with respect tochange in the control parameter P. In addition, the amount of change inthe lighting ratio R can be decreased as the control parameter P becomeslarger. Accordingly, by using algorithm which determines a displaymethod and a peripheral circuit which makes many control modes to beselected, more suitable control modes in accordance with the controlparameter P can be realized. Therefore, higher-quality display with fewmotion blur and flicker can be obtained.

Here, change in the lighting ratio R and the average luminance B withrespect to the control parameter P is summarized. When a condition thatthe product of the lighting ratio R and the average luminance B isconstant is satisfied, graphs shown in FIGS. 5A to 5D each describe arelationship between the lighting ratio R and the average luminance B.

Each of FIGS. 5A to 5C is a graph in which a horizontal axis representsthe control parameter P and a vertical axis logarithmically shows aratio of the lighting ratio R with respect to R₀ or a ratio of theaverage luminance B with respect to B₀. Here, R₀ and B₀ are values ofR(P)/R₀ and B(P)/B₀ when P=0. When R(P)/R₀ and B(P)/B₀ are expressed bya graph in which such axes are used, R(P)/R₀ and B(P)/B₀ have symmetricshapes about a linear line corresponding to 1 in the vertical axis. Thatis, a product of R(P)/R₀ and B(P)/B₀ is 1 regardless of a value of thecontrol parameter P. This can be led from the fact that the product ofR(P)/R₀ and B(P)/B₀ is R₀B₀ and is constant regardless of P when P=0.

The above-described characteristics are briefly described below. Forexample, the case in which a value of R(P_(X))/R₀ is 10^(X) isconsidered (X is a real number). At this time, a value of B(P_(X))/B₀ is1/10^(X)=10^(−X). Here, R(P_(X))/R₀ and B(P_(X))/B₀ are plotted in agraph of a logarithmic axis. At this time, when it is noted that alocation in the logarithmic axis is just a value of an exponent, as fora location at which 10^(X) is plotted and a location at which 10^(−X) isplotted, a distance of both positions from 10⁰=1 is the absolute valueof X. That is, a midpoint of line segments combining R(P_(X))/R₀ andB(P_(X))/B₀ is 1. Since this characteristic is applied to all P, it canbe concluded that R(P)/R₀ and B(P)/B₀ have symmetric shapes about thelinear line corresponding to 1 in the vertical axis.

FIG. 5A is a graph showing the case where the lighting ratio R decreaseslinearly with respect to the control parameter P. At this time, theaverage luminance B increases linearly with respect to the controlparameter P. In addition, R(P)/R₀ and B (P)/B₀ have symmetric shapesabout a linear line of R(P)/R₀=B(P)/B₀=1.

FIG. 5B is a graph showing the case where the lighting ratio R decreasesas shown by an upward curving line with respect to the control parameterP. At this time, the average luminance B increases as shown by adownward curving line with respect to the control parameter P. Inaddition, R(P)/R₀ and B(P)/B₀ have symmetric shapes centering on alinear line of R(P)/R₀=B(P)/B₀=1.

FIG. 5C is a graph showing the case where the lighting ratio R decreasesas shown by a downward curving line with respect to the controlparameter P. At this time, the average luminance B increases as shown byan upward curving line with respect to the control parameter P. Inaddition, R(P)/R₀ and B(P)/B₀ have symmetric shapes about a linear lineof R(P)/R₀=B(P)/B₀=1.

From the condition that the product of the lighting ratio R and theaverage luminance B in this embodiment mode is always constant in thismanner, the graph where change in the lighting ratio R and the averageluminance B with respect to the control parameter P has a symmetricshape about 1 in a symmetric axis. Thus, fluctuation in the integratedluminance L can be decreased, so that it is not perceived as fluctuationin luminance by human eyes even when the control parameter is greatlychanged. Therefore, a display device with little flicker can beobtained.

Next, another control modes of the lighting ratio R and the averageluminance B are described with reference to FIGS. 6A to 6P. Here, sincethe control mode of the average luminance B can be almost unambiguouslydetermined by the control mode of the lighting ratio R, description ofthe control mode of the average luminance B is omitted hereinafter andthe control mode of the lighting ratio R is only described. Note thatalthough the description is omitted, it is preferable that the averageluminance B also be controlled by the above-described method.

FIGS. 6A to 6P each show a method in which the control parameter P isdivided into two regions (a region 1 and a region 2) and the lightingratio R is controlled by the above-described mode in each region. Here,a region where the control parameter P is small is referred to as theregion 1 and a region where the control parameter P is large is referredto as the region 2.

First, the case where a value of the lighting ratio R is constant in theregion 1 is described. In this case, R(P)=R₀ is satisfied in theregion 1. This is because P=0 and R(0)=R₀ is satisfied in the region 1.In addition, in this case, at least four control modes are conceivablein the region 2. That is, the four control modes correspond to the casewhere R (P) in the region 2 is constant (see FIG. 6A), the case where R(P) in the region 2 decreases linearly (see FIG. 6B), the case where R(P) in the region 2 decreases as shown by an upward curving line (seeFIG. 6C), and the case where R (P) in the region 2 decreases as shown bya downward curving line (see FIG. 6D).

Next, the case where a value of the lighting ratio R decreases linearlyin the region 1 is described. In this case, the value of the lightingratio R decreases linearly from R(0)=R₀ as a starting point in theregion 1. In addition, in this case, at least four control modes areconceivable in the region 2. That is, the four control modes correspondto the case where R (P) in the region 2 is constant (see FIG. 6E), thecase where R (P) in the region 2 decreases linearly (see FIG. 6F), thecase where R (P) in the region 2 decreases as shown by an upward curvingline (see FIG. 6G), and the case where R (P) in the region 2 decreasesas shown by a downward curving line (see FIG. 6H).

Next, the case where a value of the lighting ratio R decreases as shownby an upward curving line in the region 1 is described. In this case,the value of the lighting ratio R decreases as shown by the upwardcurving line from R(0)=R₀ as a starting point in the region 1. Inaddition, in this case, at least four control modes are conceivable inthe region 2. That is, the four control modes correspond to the casewhere R (P) in the region 2 is constant (see FIG. 6I), the case where R(P) in the region 2 decreases linearly (see FIG. 6J), the case where R(P) in the region 2 decreases as shown by an upward curving line (seeFIG. 6K), and the case where R (P) in the region 2 decreases as shown bya downward curving line (see FIG. 6L).

Next, the case where a value of the lighting ratio R decreases as shownby a downward curving line in the region 1 is described. In this case,the value of the lighting ratio R decreases as shown by the downwardcurving line from R(0)=R₀ as a starting point in the region 1. Inaddition, in this case, at least four control modes are conceivable inthe region 2. That is, the four control modes correspond to the casewhere R (P) in the region 2 is constant (see FIG. 6M), the case where R(P) in the region 2 decreases linearly (see FIG. 6N), the case where R(P) in the region 2 decreases as shown by an upward curving line (seeFIG. 6O), and the case where R (P) in the region 2 decreases as shown bya downward curving line (see FIG. 6P).

When a value of the lighting ratio R is constant in each region, it isnot necessary that the control modes be changed precisely with respectto change in the control parameter P. Accordingly, since algorithm whichdetermines a display method and a peripheral circuit which makes manycontrol modes to be selected are not needed, manufacturing cost of thedisplay device can be reduced. In addition, since the size of a circuitand frequency of operation can be reduced, power consumption can bereduced.

When a value of the lighting ratio R decreases linearly in each region,the control modes can be changed precisely with respect to change in thecontrol parameter P. Accordingly, by using algorithm which determines adisplay method and a peripheral circuit which makes many control modesto be selected, suitable control modes in accordance with the controlparameter P can be realized. Therefore, high-quality display with littlemotion blur and little flicker can be obtained.

When a value of the lighting ratio R decreases as shown by an upwardcurving line in each region, the control modes can changed finely withrespect to change in the control parameter P. In addition, the amount ofchange in the lighting ratio R can be increased as the control parameterP becomes larger. Accordingly, by using algorithm which determines adisplay method and a peripheral circuit which makes many control modesto be selected, more suitable control modes in accordance with thecontrol parameter P can be realized. Therefore, higher-quality displaywith little motion blur and flicker can be obtained.

When a value of the lighting ratio R decreases as shown by a downwardcurving line in each region, the control modes can be changed preciselywith respect to change in the control parameter P. In addition, theamount of change in the lighting ratio R can be decreased as the controlparameter P becomes larger Accordingly, by using algorithm whichdetermines a display method and a peripheral circuit which makes manycontrol modes to be selected, more suitable control modes in accordancewith the control parameter P can be realized. Therefore, higher-qualitydisplay with little motion blur and little flicker can be obtained.

In the control mode where the control parameter P is divided into thetwo regions (the region 1 and the region 2), it is important that R (P)can have discontinuous values at a boundary between different regions.When a difference in values at the boundary between the differentregions is small, the control mode has an advantage in that a displaydefect (e.g., an unnatural contour or a flicker) due to drastic changein the control mode hardly occurs because change in R (P) with respectto change in P at the vicinity of the boundary is small.

When a difference in values at the boundary between the differentregions is large, the control mode has an advantage in that an emphaticeffect on display due to drastic change in the control mode is large andsharp display can be performed because change in R (P) with respect tochange in P in the vicinity of the boundary is large.

Here, the number of regions obtained by dividing the control parametermay be more than two. For example, the control parameter P may bedivided into three regions or may be divided into three or more regions.By dividing the control parameter P into three or more regions, morevarious control modes can be realized. In particular, R (P) can havediscontinuous values and the number of boundaries of different regionsis increased, which is important. That is, in each region, more variouscontrol modes can be realized in the case where R (P) decreases linearlywith respect to the control parameter P, the case where R (P) decreasesas shown by an upward curving line with respect to the control parameterP, and the case where R (P) decreases as shown by a downward curvingline with respect to the control parameter P. In addition to this, evenin the case where R (P) is constant with respect to the controlparameter P in each region, it is particularly advantageous that acertain number of control modes can be obtained. That is, advantages ofa simple circuit (e.g., reduction in manufacturing cost and reduction inpower consumption) and advantages of realization of various controlmodes are compatible.

This mode can be understood with reference to FIGS. 7A to 7E. FIG. 7Ashows the case where the control parameter P are divided into threeregions (a region 1, a region 2, and a region 3) and R (P) is constantin each region.

FIG. 7B shows the case where the control parameter P are divided intothree regions (a region 1, a region 2, and a region 3) and R (P)decreases linearly in each region.

FIG. 7C shows the case where the control parameter P are divided intothree regions (a region 1, a region 2, and a region 3) and R (P)decreases as shown by an upward curving line in each region.

FIG. 7D shows the case where the control parameter P are divided intothree regions (a region 1, a region 2, and a region 3) and R (P)decreases as shown by a downward curving line in each region.

Here, it is obvious that combinations of modes of R (P) in each regionare not limited to the combinations shown in FIGS. 7A to 7E. Needles tosay, these combinations are included in the control modes in thisembodiment mode; however, the combinations are omitted here, and thecase where the modes of R (P) in each region are the same is typicallydescribed.

FIG. 7E shows the case where the control parameter P is divided into n(n is a positive integer) pieces of regions (a region 1, a region 2, aregion 3, . . . , and a region n) and R (P) is constant in each region.When n is a certain number (approximately 5 to 15), advantages of asimple circuit (e.g., reduction in manufacturing cost and reduction inpower consumption) and advantages of realization of various controlmodes, which are described above, are compatible.

Note that a mode where the lighting ratio R and the average luminance Bare changed with respect to the control parameter P may be a mode whichcan be selected from a plurality of kinds. That is, a plurality ofdifferent R (P) and B (P) may be prepared in advance, and a secondcontrol parameter Q which is prepared separately from the controlparameter P may determine which R (P) and B (P) to be used. At thistime, the lighting ratio R and the average luminance B are denoted byR_(Q) (P) and B_(Q) (P) respectively, and the control parameter P isreferred to as a first parameter for convenience. For example, when thesecond parameter Q is an integer ranging from 1 to n, the lighting ratioR and the average luminance B are referred to as R₁(P), R₂(P), . . . ,and R_(n)(P), and B₁(P), B₂(P), . . . , and B_(n)(P).

This mode can be understood with reference to FIGS. 8A to 8G. In FIGS.8A to 8G, the second parameter Q is an integer ranging from 1 to 3. FIG.8A shows the case where each of R₁(P), R₂(P), . . . , and R₃(P) isconstant with respect to the first parameter P. When the first controlparameter P is 0, R₁(0)=R₁₀, R₂(0)=R₂₀, and R₃(0)=R₃₀ are satisfied. Inthis manner, in each mode of the lighting ratio R with respect to thesecond control parameter Q, the lighting ratio R can have differentvalues from each other when the first control parameter P is 0. Thus,advantages of a simple circuit (e.g., reduction in manufacturing costand reduction in power consumption) and advantages of realization ofvarious control modes are compatible.

Note that since the mode of the average luminance B can be determinedbased on the mode of the lighting ratio R in some degree similarly toanother description in this embodiment mode, description thereof isomitted here.

In another example in which the modes of the lighting ratio R and theaverage luminance B are controlled by the first control parameter P andthe second control parameter Q, R₁ (P) is constant with respect to thefirst parameter P, R₂ (P) decreases linearly with respect to the firstcontrol parameter P, and R₃ (P) decreases linearly with respect to thefirst control parameter P. Here, a gradient of linear decrease ispreferably changed in accordance with the second control parameter Q. Inaddition, in each mode of the lighting ratio R with respect to thesecond control parameter Q, the lighting ratio R can have differentvalues from each other when the first control parameter P is 0.

This mode can be understood with reference to FIG. 8B. By controlling inthis manner, more various control modes can be realized compared withthe case where the number of control parameters is 1.

In another example in which the modes of the lighting ratio R and theaverage luminance B are controlled by the first control parameter P andthe second control parameter Q, R₁ (P) is constant with respect to thefirst parameter P, R₂ (P) decreases linearly with respect to the firstcontrol parameter P, and R₃ (P) decreases as shown by an upward curvingline with respect to the first control parameter P. Here, a ratio ofdecrease is preferably changed in accordance with the second controlparameter Q. In addition, in each mode of the lighting ratio R withrespect to the second control parameter Q, the lighting ratio R can havedifferent values from each other when the first control parameter P is0.

This mode can be understood with reference to FIG. 8C. By controlling inthis manner, more various control modes can be realized compared withthe case where the number of control parameters is 1.

In another example in which the modes of the lighting ratio R and theaverage luminance B are controlled by the first control parameter P andthe second control parameter Q, R₁ (P) decreases linearly with respectto the first parameter P, R₂ (P) decreases linearly with respect to thefirst control parameter P, and R₃ (P) decreases linearly with respect tothe first control parameter P. Here, a gradient of linear decrease ispreferably changed in accordance with the second control parameter Q. Inaddition, in each mode of the lighting ratio R with respect to thesecond control parameter Q, the lighting ratio R can have differentvalues from each other when the first control parameter P is 0.

This mode can be understood with reference to FIG. 8D. By controlling inthis manner, more various control modes can be realized compared withthe case where the number of control parameters is 1.

In another example in which the modes of the lighting ratio R and theaverage luminance B are controlled by the first control parameter P andthe second control parameter Q, R₁ (P) decreases as shown by an upwardcurving line with respect to the first control parameter P, R₂ (P)decreases as shown by an upward curving line with respect to the firstcontrol parameter P, and R₃ (P) decreases as shown by an upward curvingline with respect to the first control parameter P. Here, a ratio ofdecrease is preferably changed in accordance with the second controlparameter Q. In addition, in each mode of the lighting ratio R withrespect to the second control parameter Q, the lighting ratio R can havedifferent values from each other when the first control parameter P is0.

This mode can be understood with reference to FIG. 8E. By controlling inthis manner, more various control modes can be realized compared withthe case where the number of control parameters is 1.

In another example in which the modes of the lighting ratio R and theaverage luminance B are controlled by the first control parameter P andthe second control parameter Q, R₁ (P) decreases as shown by an upwardcurving line with respect to the first control parameter P, R₂ (P)decreases as shown by an upward curving line with respect to the firstcontrol parameter P, and R₃ (P) decreases linearly with respect to thefirst control parameter P. Here, a ratio of decrease is preferablychanged in accordance with the second control parameter Q. In addition,in each mode of the lighting ratio R with respect to the second controlparameter Q, the lighting ratio R can have different values from eachother when the first control parameter P is 0.

This mode can be understood with reference to FIG. 8F. By controlling inthis manner, more various control modes can be realized compared withthe case where the number of control parameters is 1.

Note that only typical combinations are described in description of themethod in which the first control parameter P and the second controlparameter Q are used. However, various modes described in thisembodiment mode can be used for the lighting ratio R and the averageluminance B.

For example, as shown in FIG. 8G the first control parameter P isdivided into n (n is a positive integer) pieces of regions (a region 1,a region 2, a region 3, . . . , and a region n), and the lighting ratioR and the average luminance B can be combined with a method in which R(P) is constant in each region. A value of R (P) in each region ispreferably small as the second control parameter Q becomes larger. Thus,advantages of a simple circuit (e.g., reduction in manufacturing costand reduction in power consumption) and advantages of realization ofvarious control modes are compatible.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 2

In this embodiment mode, among methods in each of which the lightingratio R is changed under a condition that luminance perceived by humaneyes is constant and methods in each of which luminance perceived byhuman eyes is changed, some typical examples are described.

First, an example of a control method of the lighting ratio R isdescribed. As a control method of the lighting ratio R, (1) a method ofdirectly writing blanking data to each pixel, (2) a method of blinkingthe whole backlight, and (3) a method of sequentially blinking abacklight which is divided by areas can be mainly given.

The method (1) can be applied to both the case where a display elementincluded in a display device is a self-luminous element typified by anelement included in an EL display, a PDP, or an EFD and the case where adisplay element included in a display device is a non-light emittingelement typified by an element included in a liquid crystal display. Themethods (2) and (3) can be applied to the case where a display elementincluded in a display device is a non-light emitting element.

Before the control method of the lighting ratio R is described, astructure of pixels included in an active matrix display device isdescribed. FIG. 9G shows a structural example of a pixel included in anactive matrix display device.

The pixel included in the active matrix display device includes a pixelregion, a switching means, a display element, a signal holding means, asignal transmitting means, and a switch controlling means. A pixelregion 900, a switching means 901, a display element 902, a signalholding means 904, a signal transmitting means 906, and a switchcontrolling means 907 are included in the structural example of thepixel shown in FIG. 9G. However, the invention is not limited to this,and various structures can be used for the display device. For example,a structure such as a passive matrix structure, an MIM (metal insulatormetal) structure, or a TFD (thin film diode) structure may be used.

In FIG. 9G, more specifically, the switching means 901 is a transistor.The display element 902 is a liquid crystal element (hereinafter alsoreferred to as the liquid crystal element 902). The signal holding means904 is a capacitor (hereinafter also referred to as the capacitor 904).The signal transmitting means 906 is a data line (also referred to as asource line) (hereinafter referred to as the data line 906). The switchcontrolling means 907 is a scan line (also referred to as a gate line)(hereinafter referred to as the scan line 907). Note that a counterelectrode 903 for controlling the liquid crystal element 902 and acommon line 905 for fixing a potential of one of electrodes of thecapacitor 904 may be provided as necessary. Note also that the commonline may be shared with another scan line.

In a display portion of the display device, the pixel regions 900 arearranged in matrix. At this time, when the pixel regions 900 arranged ina row sideways are focused, the scan lines 907 thereof are common.Similarly, when the pixel regions 900 arranged in tandem are focused,the data lines 906 thereof are common.

That is, the number of wirings can be reduced when the data lines 906thereof are common. On the other hand, different signals cannot bewritten to the pixel regions 900 arranged in tandem concurrently. Here,the data lines 906 are used by being divided in terms of time bysequentially scanning the scan lines 907 which are common in the pixelregions 900 arranged in the row sideways, so that a different datasignal can be written to each pixel.

A mode of this sequential scanning can be understood with reference toFIG. 9A. The graph shown in FIG. 9A shows a mode of sequential scanningof the display device, in which a horizontal axis represents time and avertical axis represents a scanning direction of the pixel. Solid linesin the graph show positions in which a plurality of scan lines includedin the display device are selected. That is, in the graph shown in FIG.9A, scanning is performed sequentially from an upper scan line to alower scan line in the vertical axis when one frame period is started,and scanning of all the scan lines are completed at timing at which oneframe period is completed. Note that a scanning order is not limited tothis and scanning may be performed sequentially from a lower scan lineto an upper scan line in the vertical axis; however, the case wherescanning is performed sequentially from an upper scan line to a lowerscan line is described typically in this embodiment mode.

The mode of sequential scanning shown in FIG. 9A corresponds to the casewhere a data signal is written to each pixel once in one frame period.At this time, all the pixels continuously emit light with luminance inaccordance with the written data signals in one frame period. That is,the image display period τ_(a)=F (F is length of one frame period).Therefore, the lighting ratio R at this time is 1 from Formula 1.

Next, a mode of sequential scanning when the lighting ratio R is smallerthan 1 is described. As for a method of directly writing blanking datato each pixel, after a specific data signal is written to each pixel, itis necessary that the signal written to the pixel be rewritten into asignal in accordance with blanking data at appropriate timing.

A mode of sequential scanning at this time can be understood withreference to FIGS. 9B to 9F. The graph shown in FIG. 9B shows a mode ofsequential scanning of the display device when the lighting ratio R=1/2.Solid lines in the diagram show timing of data writing scanning forwriting a specific data signal to each pixel. In addition, broken linesin the diagram show timing of blanking writing scanning for controllingthe lighting ratio R. When the lighting ratio R=1/2 is realized in thismanner, it is only necessary to start blanking writing scanning whentime of F/2 passes from timing at which data writing scanning isstarted. Then, a period after blanking writing scanning is performed anduntil data writing scanning of the next frame is performed correspondsto a blanking display period.

Similarly, when the lighting ratio R=1/3 is realized in this manner, itis only necessary to start blanking writing scanning when time of F/3passes from timing at which data writing scanning is started. At thistime, since the display period τ_(a)=F/3, the lighting ratio R at thistime is 1/3 from Formula 1. A mode of sequential scanning at this timecan be understood with reference to FIG. 9C.

Similarly, when the lighting ratio R=1/4 is realized in this manner, itis only necessary to start blanking writing scanning when time of F/4passes from timing at which data writing scanning is started. At thistime, since the display period τ_(a)=F/4, the lighting ratio R at thistime is 1/4 from Formula 1. A mode of sequential scanning at this timecan be understood with reference to FIG. 9D.

Similarly, when the lighting ratio R=2/3 is realized in this manner, itis only necessary to start blanking writing scanning when time of 2F/3passes from timing at which data writing scanning is started. At thistime, since the display period τ_(a)=2F/3, the lighting ratio R at thistime is 2/3 from Formula 1. A mode of sequential scanning at this timecan be understood with reference to FIG. 9E.

Similarly, when the lighting ratio R=3/4 is realized in this manner, itis only necessary to start blanking writing scanning when time of 3F/4passes from timing at which data writing scanning is started. At thistime, since the display period τ_(a)=3F/4, the lighting ratio R at thistime is 3/4 from Formula 1. A mode of sequential scanning at this timecan be understood with reference to FIG. 9F.

Values of the lighting ratio R can be set variously in accordance withwriting timing of blanking data in this manner.

Here, it should be noted that when the lighting ratio R is controlled byperforming blanking writing at specific timing after data writingscanning is performed, there is a period during which data writingscanning and blanking writing scanning are performed at the same time.That is, when certain time of each of the graphs shown in FIGS. 9B to 9Fis focused, data writing scanning and blanking writing scanning coincideat different positions.

Even when data writing scanning and blanking writing scanning coincideat different positions in this manner, there are a plurality of methodsfor accurately writing a signal in each scanning. For example, there isa method in which a period during which one scan line is selected (onegate election period) is further divided into a plurality of periods anddata writing scanning and blanking writing scanning are assigned to eachperiod. The structure shown in FIG. 9G can be used for a structure ofthe pixel region included in the display device at this time. Therefore,the lighting ratio R can be controlled variously without changing thepixel structure.

As another method, there is a method of adding a signal line and aswitching element which are dedicated for blanking writing scanning tothe pixel region. By using this method, signals can be writtenaccurately by each scanning without dividing one gate selection period.FIG. 9H shows a structural example of a pixel included in such an activematrix display device.

The pixel included in the active matrix display device to which thesignal line and the switching element which are dedicated for blankingwriting scanning are added includes a pixel region, a first switchingmeans, a second switching means, a display element, a signal holdingmeans, a first signal transmitting means, a second signal transmittingmeans, a first switch controlling means, and a second switch controllingmeans. A pixel region 910, a first switching means 911, a secondswitching means 918, a display element 912, a signal holding means 914,a first signal transmitting means 916, a second signal transmittingmeans 920, a first switch controlling means 917, and a second switchcontrolling means 919 are included in the structural example of thepixel shown in FIG. 9H.

In FIG. 9H, more specifically, the first switching means 911 and thesecond switching means 918 are transistors. The display element 912 is aliquid crystal element (hereinafter also referred to as the liquidcrystal element 912). The signal holding means 914 is a capacitor(hereinafter also referred to as the capacitor 914). The first signaltransmitting means 916 is a data line (also referred to as a sourceline). The second signal transmitting means 920 is a blanking signalline (hereinafter also referred to as the blanking signal line 920). Thefirst switch controlling means 917 is a writing scan line. The secondswitch controlling means 919 is a blanking scan line. Note that acounter electrode 913 for controlling the liquid crystal element 912 anda common line 915 for fixing a potential of one of electrodes of thecapacitor 914 may be provided as necessary. Note also that the blankingsignal line may be shared with the common line, a writing scan line ofanother pixel, and the blanking scan line.

In addition, a driving method of a display device in accordance withthis document can be used for both the case where a liquid crystalelement is normally black and the case where a liquid crystal element isnormally white. Here, normally black corresponds to a mode where a blackimage is displayed when voltage is not applied to a liquid crystalelement. Normally white is a mode where a white image is displayed whenvoltage is not applied to a liquid crystal element. Note that a methodin accordance with this document can also be applied to a normally-whiteliquid crystal element by inverting polarity of signal voltage even whenthe signal voltage is shown as normally black.

By using the pixel structure to which the signal line and a switchingelement which are dedicated for blanking writing scanning are added inthis manner, a signal can be written accurately by each scanning withoutdividing one gate selection period. Therefore, driving frequency of aperipheral circuit can be suppressed low, so that power consumption canbe reduced.

Next, specific operating methods of the method in which one gateelection period is further divided into a plurality of periods and datawriting scanning and blanking writing scanning are assigned to eachperiod, and the method of adding the signal line and a switching elementwhich are dedicated for blanking writing scanning are described.

First, the method in which one gate election period is divided into aplurality of regions and data writing scanning and blanking writingscanning are assigned to each period is described with reference to FIG.10A.

FIG. 10A is a diagram for describing driving conditions of a data lineand a scan line in connection with a display condition of a displayportion of a display device. A display portion 1000 includes pixelregions arranged in matrix and performs various kinds of display. Thepixel regions in FIG. 10A are similar to the structure shown in FIG. 9G.A scan line 1001 is a scan line which performs blanking writing attiming shown in FIG. 10A. A scan line 1002 is a scan line which performsdata writing at timing shown in FIG. 10A. A data line driver 1003 is acircuit which generates a signal written to each pixel in accordancewith a data signal. In FIG. 10A, the signal written to each pixel is avoltage signal, and a specific example of the voltage signal is shownabove the data line driver 1003. A scan line driver 1004 is a circuitfor driving a plurality of scan lines. Waveforms of voltage input to thescan line 1001 and the scan line 1002 from the scan line driver 1004 areshown on the left of the scan line driver 1004.

Timing for driving the scan line 1002 by the scan line driver 1004 shownin FIG. 10A is a period from time t₁ to time t₂. At this time, the dataline outputs voltage V_(data1). The voltage V_(data1) is voltage whichshould be written to a pixel selected by the scan line 1002 at timingshown in FIG. 10A.

The scan line driver 1004 drives the scan line 1001 from the time t₂ totime t₃. At this time, the data line outputs voltage V_(blank). Thevoltage V_(blank) is voltage supplying luminance which should bedisplayed in a blanking interval.

A period from the time t₁ to the time t₃ in the description heretoforecorresponds to one gate selection period under a driving condition ofthe lighting ratio R=1 without providing a blanking interval. That is,one gate selection period (a period from the time t₁ to the time t₃) isdivided into two periods (a period from the time t₁ to the time t₂ and aperiod from the time t₂ to the time t₃) and data writing scanning andblanking writing scanning are assigned to each period.

The scan line driver 1004 drives a scan line which is next to the scanline 1002 from the time t₃ to time t₄. At this time, the data lineoutputs voltage V_(data2). The voltage V_(data2) is voltage which shouldbe written to a pixel selected by the scan line which is next to thescan line 1002 at timing shown in FIG. 10A.

The scan line driver 1004 drives a scan line which is next to the scanline 1001 from the time t₄ to time t₅. At this time, the data lineoutputs voltage V_(blank). The voltage V_(blank) is voltage supplyingluminance which should be displayed in the blanking interval.

By repeating driving which is described above, signals can be accuratelywritten in each scanning even when data writing scanning and blankingwriting scanning coincide at different positions.

Note that voltage of the data line is an example for describing thedriving method, and voltage of V_(blank), V_(data1), and V_(data2) isnot limited to the voltage shown in FIG. 10A and can have variousvalues.

Next, the method of adding a signal line and a switching element whichare dedicated for blanking writing scanning to the pixel region isdescribed with reference to FIG. 10B.

FIG. 10B is a diagram for describing driving conditions of a data lineand a scan line in connection with a display condition of a displayportion of a display device. A display portion 1010 includes pixelregions arranged in matrix and performs various kinds of display. Thepixel regions in FIG. 10B are similar to the structure shown in FIG. 9H.A blanking scan line 1011 is a blanking scan line which performsblanking writing at timing shown in FIG. 10B. A writing scan line 1012is a scan line which performs data writing at timing shown in FIG. 10B.A data line driver 1013 is a circuit which generates a signal written toeach pixel in accordance with a data signal. In FIG. 10B, the signalwritten to each pixel is a voltage signal, and a specific example of thevoltage signal is shown above the data line driver 1013. A writing scanline driver 1014 is a circuit for driving a plurality of writing scanlines. Waveforms of voltage input to the writing scan line 1012 from thewriting scan line driver 1014 are shown on the left of the writing scanline driver 1014. A blanking scan line driver 1015 is a circuit fordriving a plurality of blanking scan lines. Waveforms of voltage inputto the blanking scan line 1011 from the blanking scan line driver 1015are shown on the right of the blanking scan line driver 1015.

Timing for driving the writing scan line 1012 by the writing scan linedriver 1014 shown in FIG. 10B is a period from time t₁ to time t₂. Atthis time, the data line outputs voltage V_(data1). The voltageV_(data1) is voltage which should be written to a pixel selected by thewriting scan line 1012 at timing shown in FIG. 10B.

The blanking scan line driver 1015 operates concurrently and drives theblanking scan line 1011 from the time t₁ to the time t₃. At this time, asignal written to a pixel selected by the blanking scan line 1011 attiming shown in FIG. 10B follows the voltage V_(blank) which is suppliedto the blanking signal line 920 in the pixel structure shown in FIG. 9H.

The writing scan line driver 1014 drives a writing scan line which isnext to the writing scan line 1012 from the time t₃ to time t₅. At thistime, the data line outputs voltage V_(data2). The voltage V_(data2) isvoltage which should be written to a pixel selected by the writing scanline which is next to the writing scan line 1012 at timing shown in FIG.10B.

The blanking scan line driver 1015 operates concurrently and drives ablanking scan line which is next to the blanking scan line 1011 from thetime t₃ to the time t₅. At this time, a signal written to a pixelselected by the blanking scan line which is next to the blanking scanline 1011 at timing shown in FIG. 10B follows the voltage V_(blank)which is supplied to the blanking signal line 920 in the pixel structureshown in FIG. 9H.

A period from the time t₁ to the time t₃ in the description heretoforecorresponds to one gate selection period under a driving condition ofthe lighting ratio R=1 without providing a blanking interval. That is,data writing scanning and blanking writing scanning can be performedconcurrently without dividing one gate selection period into twoperiods.

By repeating driving which is described above, a signal can beaccurately written in each scanning even when data writing scanning andblanking writing scanning coincide at different positions.

Note that voltage of the data line is an example for describing thedriving method, and voltage of V_(data1) and V_(data2) is not limited tothe voltage shown in FIG. 10B and can have various values.

Next, another mode of sequential driving when the lighting ratio R issmaller than 1 is described. As for a method of directly writingblanking data to each pixel, after a specific data signal is written toeach pixel, it is necessary that the signal written to the pixel berewritten to a signal in accordance with blanking data at appropriatetiming. Therefore, in the methods shown in FIGS. 9A to 9H, and FIGS. 10Aand 10B, it is necessary that writing scanning and blanking scanning beperformed concurrently by adding a signal line and a switching elementto a pixel region, or one gate selection period be divided into aplurality of periods and data writing and blanking writing be assignedto each period.

A method shown below is a method in which writing scanning and blankingscanning are completed in time shorter than one frame period F. By usingthis method, data writing scanning and blanking writing scanning can beperformed without either dividing one gate selection period or adding asignal line and a switching element to a pixel region.

There are a plurality of modes of the method in which writing scanningand blanking scanning are completed in time shorter than the one frameperiod F. One mode is a mode in which a period during which writingscanning and blanking scanning are completed is changed in accordancewith a value of the lighting ratio R. Here, a period during whichwriting scanning and blanking scanning are completed is referred to asτ_(w).

In the mode in which τ_(W) is changed in accordance with a value of thelighting ratio R, τ_(w) is conformed to a period having a smaller valuebetween the image display period τ_(a) and the blanking interval τ_(b),which lead the lighting ratio R. A mode of sequential scanning in thismethod can be understood with reference to FIGS. 11A, 11C, 11E, 11G,11I, and 11J. Here, each of graphs shown in FIGS. 11A to 11J shows amode of sequential scanning of the display device, in which a horizontalaxis represents time and a vertical axis represents a scanning directionof a pixel. A form of the graphs is similar to those of FIGS. 9A to 9F.

When the blanking interval τ_(b) is 0, blanking scanning is notperformed. A mode of sequential scanning at this time can be understoodwith reference to FIG. 11A. That is, sequential scanning is performed bysetting τ_(w) as F. At this time, the lighting ratio R is 1.

When τ_(a)=τ_(b)=F/2, sequential scanning is performed by setting τ_(w)as F/2. A mode of sequential scanning at this time can be understoodwith reference to FIG. 11C. That is, blanking scanning is started rightafter writing scanning is completed in a period of F/2, and blankingscanning is completed when one frame period is completed. At this time,the lighting ratio R is 1/2.

When τ_(a)=F/3 and τ_(b)=2F/3, sequential scanning is performed bysetting τ_(w) as F/3. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11E. That is, blanking scanning isstarted right after writing scanning is completed in a period of F/3,and blanking scanning is completed at time 2F/3. At this time, thelighting ratio R is 1/3.

When τ_(a)=2F/3 and τ_(b)=F/3, sequential scanning is performed bysetting τ_(w) as F/3. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11G. That is, blanking scanning isstarted at the time 2F/3 in the period of F/3 after writing scanning iscompleted in the period of F/3. Then, blanking scanning is completedwhen one frame period is completed. At this time, the lighting ratio Ris 2/3.

When τ_(a)=F/4 and τ_(b)=3F/4, sequential scanning is performed bysetting τ_(w) as F/4. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11I. That is, blanking scanning isstarted right after writing scanning is completed in a period of F/4,and blanking scanning is completed at time F/2. At this time, thelighting ratio R is 1/4.

When τ_(a)=3F/4 and τ_(b)=F/4, sequential scanning is performed bysetting τ_(w) as F/3. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11J. That is, blanking scanning isstarted at time 3F/4 in the period of F/2 after writing scanning iscompleted in the period of F/4. Then, blanking scanning is completedwhen one frame period is completed. At this time, the lighting ratio Ris 3/4.

The mode in which τ_(W) is changed in accordance with a value of thelighting ratio R can be realized by conforming τ_(W) to a period havinga smaller value between the image display period τ_(a) and the blankinginterval τ_(b), which lead the lighting ratio R in this manner. Sinceτ_(W) can be set to a suitable period in accordance with the value ofthe lighting ratio R in this manner, operating frequency of a peripheralcircuit such as a scan line driver or a data line driver can also be setto a suitable value which is in accordance with the value of thelighting ratio R. Accordingly, power consumption can be reduced.

Among the plurality of modes of the method in which writing scanning andblanking scanning are completed in time shorter than the one frameperiod F, a mode which is different from the above-described mode is amode in which the period τ_(w) during which writing scanning andblanking scanning are completed is completed rapidly without dependingon the value of the lighting ratio R.

In the mode in which the period during which writing scanning andblanking scanning are completed is completed earlier without dependingon the value of the lighting ratio R, τ_(w) is shortened as much aspossible. For example, τ_(w) is set to F/4 which is 1/4 of the one frameperiod F. A mode of sequential scanning at this time can be understoodwith reference to FIGS. 11B, 11D, 11F, 11H, 11I, and 11J.

When the blanking interval τ_(b) is 0, blanking scanning is notperformed. A mode of sequential scanning at this time can be understoodwith reference to FIG. 11B. That is, sequential scanning is performed bysetting τ_(w) as F/4. At this time, the lighting ratio R is 1.

When τ_(a)=τ_(b)=F/2, sequential scanning is performed also by settingτ_(w) as F/4. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11D. That is, blanking scanning isstarted at the time F/2 in the period of F/4 after writing scanning iscompleted in the period of F/4. Then, blanking scanning is completed atthe time 3F/4. At this time, the lighting ratio R is 1/2.

When τ_(a)=F/3 and τ_(b)=2F/3, sequential scanning is performed also bysetting τ_(w) as F/4. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11F. That is, blanking scanning isstarted at the time F/3 in a period of F/12 after writing scanning iscompleted in the period of F/4. T blanking scanning is completed at time7F/12. At this time, the lighting ratio R is 1/3.

When τ_(a)=2F/3 and τ_(b)=F/3, sequential scanning is performed also bysetting τ_(w) as F/4. A mode of sequential scanning at this time can beunderstood with reference to FIG. 11H. That is, blanking scanning isstarted at the time 2F/3 in a period of 5F/12 after writing scanning iscompleted in the period of F/4. Then, blanking scanning is completed attime 11F/12. At this time, the lighting ratio R is 2/3.

When τ_(a)=F/4 and τ_(b)=3F/4, sequential scanning is performed also bysetting τ_(w) as F/4. The mode of sequential scanning at this time canbe understood with reference to FIG. 11I. That is, blanking scanning isstarted right after writing scanning is completed in the period of F/4.Then, blanking scanning is completed at the time F/2. At this time, thelighting ratio R is 1/4.

When τ_(a)=3F/4 and τ_(b)=F/4, sequential scanning is performed also bysetting as F/3. The mode of sequential scanning at this time can beunderstood with reference to FIG. 11J. That is, blanking scanning isstarted at time 3F/4 in the period of F/2 after writing scanning iscompleted in the period of F/4. Then, blanking scanning is completedwhen one frame period is completed. At this time, the lighting ratio Ris 3/4.

Here, in the mode in which τ_(w) is completed earlier without dependingon the value of the lighting ratio R, a mode with a lighting ratio otherthan those shown in FIGS. 11B, 11D, 11F, 11H, 11I, and 11J can be easilyrealized. That is, a period at which blanking scanning is started can beset freely, so that a mode with a lighting ratio corresponding to it canbe realized. In addition, a range of the image display period τ_(a)which can be set is equal to or greater than and equal to or less than1−τ_(w). For example, in an example in which τ_(w) is set as F/4, thelighting ratio R can be freely selected in a range of equal to orgreater than 1/4 and equal to or less than 3/4.

Note that the mode in which the period τ_(w) during which writingscanning and blanking scanning are completed is changed in accordancewith the value of the lighting ratio R and the mode in which iscompleted earlier without depending on the value of the lighting ratio Rcan be combined. For example, when the lighting ratio R can be freelyselected in a range of equal to or greater than 1/3 and equal to or lessthan 2/3, τ_(W) is set as F/3. Then, when the lighting ratio R is neededto be selected in a range of greater than that, τ_(w) is set to besmaller than F/3. For example, when τ_(w) is set as F/4, the lightingratio R in a range of equal to or greater than 1/4 and equal to or lessthan 1/3 and the lighting ratio R in a range of equal to or greater than2/3 and equal to or less than 3/4, which cannot be selected when τ_(w)is set as F/3, can be selected. Since the value of the lighting ratio Rcan be selected in a certain range in this manner and operatingfrequency of the peripheral circuit such as a scan line driver or a dataline driver can be set to a suitable value which is in accordance withthe range of the value of the lighting ratio R, power consumption can bereduced, which is extremely advantageous.

(1) a method of directly writing blanking data to each pixel, (2) amethod of blinking the whole backlight, and (3) a method of sequentiallyblinking a backlight which is divided by areas can be mainly given as acontrol method of the lighting ratio R, which has been described at thebeginning of this embodiment mode. The driving method which is describedheretofore is a method which can be used for the method (1).

The method (1) can be applied to both of the case where a displayelement included in a display device is a self-luminous element typifiedby an element included in an EL display, a PDP, or an EFD and the casewhere a display element included in a display device is a non-lightemitting element typified by an element included in a liquid crystaldisplay. Next, driving methods of the methods (2) and (3) are described.

The method (2) where the whole backlight is blinked can be used when adisplay device includes a member called a backlight. A backlightcorresponds to a light source provided on the back of a display portionof a display device. In particular, a backlight is advantageous when adisplay portion of a display device includes a non-light emittingdisplay element. As such a display element, a transmissive liquidcrystal element and a semi-transmissive liquid crystal element can begiven, for example. Note that a display device may include a front lightprojector, a rear and front projector, or a light source for a projectorwithout limiting to a backlight.

In the case of a non-light emitting display element, a light source isnecessary in addition to the display element because the display elementdoes not emit light by itself. At this time, a backlight is used in somecases. A backlight is usually a surface light source which uniformlyilluminates a display portion of a display device. At this time, thedisplay element has a function of determining how much light of thislight source transmits. Accordingly, increase and decrease in luminanceof the backlight corresponds to increase and decrease in brightness ofthe whole image.

That is, in a display device which includes a backlight, a blankinginterval can be provided by changing luminance of the backlight withoutwriting a blanking signal to a display element. Further, the lightingratio R can be controlled by controlling the length of a period duringwhich the luminance of the backlight is changed.

FIGS. 12A and 12B each show a mode of a method in which the lightingratio R is controlled by controlling luminance of a backlight. Each ofgraphs shown in FIGS. 12A and 12B shows a mode of sequential scanning ofa display device and timing at which the backlight is controlled at thesame time, in which a horizontal axis represents time and a verticalaxis represents a scanning direction of a pixel. Solid lines in thegraphs show positions in which a plurality of scan lines included in thedisplay device are selected.

In the method in which the lighting ratio R is controlled by controllingthe luminance of the backlight, switching of the luminance of thebacklight and writing scanning are preferably controlled at timing whichis different in terms of time. This is because by controlling switchingof the luminance of the backlight and writing scanning at timing whichis different in terms of time, all the pixels are classified into pixelswhich do not emit light before data is written and do not emit lightright after data is written or pixels which emit light before data iswritten and continuously emit light when data is written. Therefore,since pixels in different conditions are not displayed concurrently inthe display portion, a problem such as display unevenness can bereduced. This mode can be understood with reference to the graph shownin FIG. 12A. In FIG. 12A, a period during which the luminance of thebacklight is changed right after writing scanning is completed isprovided in one frame period. The period corresponds to a region shownby a slanted line in FIG. 12A. When an image is displayed by lightingthe backlight in the period, the period corresponds to an image displayperiod. Alternatively, when blanking is displayed by turning out thebacklight or reducing light of the backlight in the period, the periodcorresponds to a blanking interval.

In addition, the length of the period during which the luminance of thebacklight is changed may be changed. FIG. 12B shows an example thereof.In FIG. 12B, a period during which luminance of a backlight is changedis shorter than a similar period in FIG. 12A. In this manner, thelighting ratio R can be controlled by the length of the period duringwhich the luminance of the backlight is changed.

When the lighting ratio R is controlled by the method of blinking thewhole backlight, the backlight is turned out or light of the backlightis reduced in the blanking interval, so that there is an advantage inthat power consumption can be reduced to a corresponding extent. Inaddition, since a structure of a circuit is simple, manufacturing costcan be reduced.

Next, among the control methods of the lighting ratio R, the method (3)in which a backlight which is divided by areas is sequentially blinkedis described. Here, an example in which the backlight is divided byareas in a direction which is parallel to a scan line is described.

FIGS. 13A to 13C each show a mode of a method in which the lightingratio R is controlled by controlling luminance of a backlight which isdivided by areas. Each of graphs shown in FIGS. 13A to 13C shows a modeof sequential scanning of a display device and timing at which thebacklight is controlled at the same time, in which a horizontal axisrepresents time and a vertical axis represents a scanning direction of apixel. Solid lines in the graphs show positions in which a plurality ofscan lines included in the display device are selected.

In the method in which the lighting ratio R is controlled by controllingthe luminance of the backlight which is divided by areas, switching ofthe luminance of the backlight and writing scanning are preferablycontrolled at timing which is different in terms of time. This isbecause by controlling switching of the luminance of the backlight andwriting scanning at timing which is different in terms of time, all thepixels are classified into pixels which do not emit light before data iswritten and do not emit light right after data is written or pixelswhich emit light before data is written and continuously emit light whendata is written. Therefore, since pixels in different conditions are notdisplayed at the same time in the display portion, a problem such asdisplay unevenness can be reduced. This mode can be understood withreference to the graphs shown in FIG. 13A to 13C.

In FIG. 13A, the case is described in which a ratio of a period duringwhich the luminance of the backlight is changed to one frame period isapproximately 2/3. The period corresponds to a region shown by a slantedline. When an image is displayed by lighting the backlight in theperiod, the period corresponds to an image display period.Alternatively, when blanking is displayed by turning out the backlightor reducing light of the backlight in the period, the period correspondsto a blanking interval.

By using the backlight which is divided by areas, a period during whichthe luminance of the backlight is changed in each area can be varied. InFIG. 13A, the backlight is divided into five areas and luminance of eacharea is sequentially controlled.

In addition, the length of the period during which the luminance of thebacklight which is divided by areas is changed may be changed. FIG. 13Bshows an example thereof. In FIG. 13B, a period during which luminanceof a backlight is changed is shorter than a similar period in FIG. 13A.In this manner, the lighting ratio R can be controlled by the length ofthe period during which the luminance of the backlight is changed.

In addition, when the backlight which is divided by areas is used,control can be performed such that switching of the luminance of thebacklight and writing scanning do not overlap with each other in termsof time without increasing scan speed of writing scanning. For example,in FIG. 13A or FIG. 13B, the period during which the luminance of thebacklight is changed can be provided even when writing scanning isperformed in the whole one frame period. Thus, operating frequency of aperipheral circuit such as a scan line driver or a data line driver canbe set small regardless of a value of the lighting ratio R. Accordingly,power consumption can be reduced.

Note that even when the backlight which is divided by areas is used,scan speed of writing scanning may be increased. Thus, a display troublecaused by variation in light-emitting time between areas can be reduced.This point can be understood with reference to the graph shown in FIG.13C. The graph shown in FIG. 13C shows an example of the case where scanspeed of writing scanning is increased. From the graph shown in FIG.13C, it can be seen that variation in light-emitting time betweenadjacent areas is less than that of the case where scan speed of writingscanning is not high (FIG. 13A or FIG. 13B) when scan speed of writingscanning is increased. When variation in light-emitting time between theadjacent areas is little, a display trouble caused by variation inlight-emitting time between areas can be reduced.

As a display trouble caused by variation in light-emitting time betweenareas, increase of false light emission caused by light leakage from theareas, increase of visibility of a boundary between the areas, or thelike can be given, for example.

When the lighting ratio R is controlled by the method of controlling theluminance of the backlight which is divided by areas, the backlight isturned out or light of the backlight is reduced in the blankinginterval, so that there is an advantage in that power consumption can bereduced to a corresponding extent.

Heretofore, a method of controlling the lighting ratio R is describedunder a condition that luminance perceived by human eyes (L/F) isconstant. A method for changing luminance perceived by human eyes isdescribed below.

In order to change luminance perceived by human eyes, there are a methodof changing the integrated luminance L and a method of changing thelighting ratio R. Here, when it is assumed that the lighting ratio R isconstant, the integrated luminance L should be changed in order tochange the luminance perceived by human eyes.

The integrated luminance L is luminance obtained by time integrating theinstantaneous luminance I (t) as shown in Formula 3. That is, it isnecessary that the instantaneous luminance I (t) be changed in order tochange the integrated luminance L.

Here, in the case where a display element included in a display deviceis a self-luminous element such as an element included in an EL display,a PDP, or an EFD, luminance of the display element itself changes theinstantaneous luminance I (t). That is, the instantaneous luminance I(t) can be changed by writing a predetermined signal to each displayelement.

On the other hand, luminance of a display element itself changes theinstantaneous luminance I (t) even in the case where the display elementincluded in a display device is a non-light emitting element; however,the luminance of the display element itself can be divided into aplurality of elements in the case where the display element is anon-light emitting element. That is, the plurality of factors correspondto backlight luminance B_(L) and transmittance T of the display element.Therefore, the luminance of the display element is a product of thebacklight luminance B_(L) and the transmittance T. The luminance of thedisplay element also corresponds to the instantaneous luminance I (t).When the description is summarized, it can be represented as Formula 8.

I(t)=B _(L)(t)T(t)  [Formula 8]

Here, Formula 8 is assigned to Formula 3 which leads the integratedluminance L. Note that when the backlight luminance B_(L) and thetransmittance T are not dependent on the time t for simplification,Formula 9 is obtained.

$\begin{matrix}{\frac{L}{F} = {B_{L}T}} & \left\lbrack {{Formula}\mspace{14mu} 9} \right\rbrack\end{matrix}$

A left-hand side of Formula 9 shows the luminance perceived by humaneyes (L/F). Therefore, when the backlight luminance B_(L) and thetransmittance T are constant, the product of B_(L) and T represents theluminance perceived by human eyes.

In a display device using a liquid crystal element, the transmittance Tis usually controlled by voltage written to a pixel and the luminanceperceived by human eyes is controlled. A numeric value in which a degreeof the luminance perceived by human eyes is represented by a positiveinteger is called a gray scale. In addition, G is used as a sign whichrepresents the gray scale. For example, when brightness between thedarkest brightness and the brightest brightness is classified into 256stages, a gray scale 0 expresses the darkest brightness and a gray scale255 expresses the brightest brightness. An intermediate gray scaleexpresses intermediate brightness of the two gray scales.

It should be noted that when a gray scale is dealt, brightness expressedby the gray scale dose not necessarily have a linear relation withphysical luminance. That is, when a relation between a gray scale andluminance is expressed by a graph, the gray scale and the luminance canbe in connection with each other by a curve having various shapes. Thiscurve showing a relationship between a gray scale and luminance iscalled a gamma curve.

A Typical gamma curve is described with reference to FIG. 14A. FIG. 14Ais a graph showing a relation between a gray scale and luminance, i.e.,a gamma curve. A horizontal axis represents a gray scale and a verticalaxis represents luminance. Here, luminance corresponds to luminanceperceived by human eyes (L/F). That is, from Formula 9, the verticalaxis represents the amount expressed by the product of B_(L) and T. Acurve 1400 shown in FIG. 14A is a gamma curve when brightness perceivedby human eyes is changed almost linearly. In this manner, an ideal gammacurve is a curve having convexity below.

When luminance B_(L)T is changed by changing a gray scale transmittanceT is usually changed. This is because although the transmittance T canbe individually controlled by changing voltage written to each pixel, itis difficult to individually control the backlight luminance B_(L)because the backlight luminance B_(L) is shared with a plurality ofpixels.

Next, a method of displaying an image normally even when the backlightluminance B_(L) decreases by controlling the transmittance T and thebacklight luminance B_(L) is described. Since the luminance B_(L)T isthe product of the transmittance T and the backlight luminance B_(L),various gamma curves can be realized by changing the transmittance T andthe backlight luminance B_(L).

A curve 1401 shown in FIG. 14A is a curve in which the transmittance Tof the curve 1400 increases in each gray scale G and is represented as afunction T₁ (G). In FIG. 14A, since the backlight luminance B_(L) is notchanged, the luminance B_(L)T is higher than the luminance of the curve1400. In addition, since the transmittance T has the maximum value andcannot be made larger than that, the curve 1401 is saturated in acertain gray scale.

A curve 1402 shown in FIG. 14B is a gamma curve at the time when thetransmittance T increases as in the curve 1401 shown in FIG. 14A and thebacklight luminance B_(L) decreases. At this time, in a region G₁₄₀₂ ofa gray scale, the transmittance T of which is saturated, there is nodifference in luminance and the curve 1402 is saturated. Luminance atthis time is denoted by a₁. In gray scale regions other than the grayscale region G₁₄₀₂, a shape of the curve 1402 preferably corresponds tothat of the curve 1400. Thus, even when power consumption is reduced bydecreasing the backlight luminance B_(L), display which is similar todisplay at the time when the backlight luminance B_(L) is not decreasedcan be performed in the gray scale regions other than the gray scaleregion G₁₄₀₂.

Note that an advantage of the method in this document is that thebacklight luminance B_(L) can be decreased by controlling the lightingratio R. Thus, power consumption of a backlight can be reduced and ablanking interval can be provided, so that motion blur can be reduced.

Here, an adverse effect on image display at the time when display isperformed in accordance with a gamma curve in which luminance issaturated as in the curve 1402 shown in FIG. 14B is described. Whenimage display is performed in accordance with a gamma curve in whichluminance is saturated as in the curve 1402, needless to say, all thegrays scales included in the gray scale region G₁₄₀₂ have the sameluminance. At this time, as an adverse effect on image display, acondition in which there is no bright gray scale, i.e., a conditioncalled blown-out highlights can be given.

However, not all the images cause blown-out highlights. In a graph shownin FIG. 14C, a horizontal axis represents the gray scale G and avertical axis represents the number of data included in the pixels. Sucha graph is called a histogram. In a histogram 1403 shown in FIG. 14C,there is almost no data in the gray scale region G₁₄₀₂. That is, as foran image originally having no data in the gray scale region G₁₄₀₂,blown-out highlights do not occur even when the curve 1402 shown in FIG.14B is used as a gamma curve.

On the other hand, a histogram 1404 shown in FIG. 14D shows the case ofan image having a certain number of data in the gray scale region G₁₄₀₂.At this time, a certain degree of blown-out highlights occurs at thetime when the curve 1402 shown in FIG. 14B is used as a gamma curve.However, when the number of data included in the gray scale region G₁₄₀₂is equal to or less than 1/10 of the total number of data, blown-outhighlights are hardly perceived.

In this manner, the method in this document analyzes a histogram of animage and determines whether the number of data of an image included ina gray scale region in which luminance is saturated is equal to or lessthan 1/10 of the total number of data. When the number of data of theimage included in the gray scale region in which the luminance issaturated is equal to or less than 1/10 of the total number of data, thetransmittance T increases such that the graph has a gamma curve which isin accordance with the function T₁ (G), and the backlight luminanceB_(L) decreases. In addition, the backlight luminance B_(L) ispreferably decreased by controlling the lighting ratio R. Thus, powerconsumption of a backlight can be reduced and a blanking interval can beprovided, so that motion blur can be reduced.

Next, the case is described in which the histogram of the image isanalyzed and the number of data of the image included in the gray scaleregion in which the luminance is saturated is equal to or greater than1/10 of the total number of data.

In the case where the histogram of the image is analyzed and the numberof data of the image included in the gray scale region in which theluminance is saturated is equal to or greater than 1/10 of the totalnumber of data, the curve 1400 is not a curve represented by thefunction T₁ (G) but a curve represented by another function when thetransmittance T of the curve 1400 increases in each of the gray scalesG.

A curve 1405 shown in FIG. 14E is a curve in which the transmittance Tof the curve 1400 increases in each of the gray scales G and isrepresented as a function T₂ (G). In FIG. 14E, since the a backlightluminance B_(L) is not changed, the luminance B_(L)T is higher than theluminance of the curve 1400. In addition, since the transmittance T hasthe maximum value and cannot be made larger than that, the curve 1405 issaturated in a certain gray scale. Here, as for a relation between thefunction T₁ (G) and the function T₂(G), T₁(G)>T₂(G) is satisfied in agray scale region in which the transmittance T is not saturated andT₁(G)=T₂(G) is satisfied in a gray scale region in which thetransmittance T is saturated.

A curve 1406 shown in FIG. 14F is a gamma curve at the time when thetransmittance T increases as in the curve 1405 shown in FIG. 14E and thebacklight luminance B_(L) decreases. At this time, in a region G₁₄₀₆ ofa gray scale, the transmittance T of which is saturated, there is nodifference in luminance and the curve 1406 is saturated. Luminance atthis time is denoted by a₂. In gray scale regions other than the grayscale region G₁₄₀₆, a shape of the curve 1406 preferably corresponds tothat of the curve 1400. Thus, even when power consumption is reduced bydecreasing the backlight luminance B_(L), display which is similar todisplay at the time when the backlight luminance B_(L) is not decreasedcan be performed in the gray scale regions other than the gray scaleregion G₁₄₀₆.

As for the gamma curve 1406 in which the transmittance T is changed inaccordance with not the function T₁ (G) but the function T₂ (G) and theluminance is made to be a₂ by decreasing the backlight luminance B_(L),the luminance is saturated in a certain gray scale region similarly tothe gamma curve 1402. However, the size of a gray scale region in whichluminance is saturated is different between the gray scale region G₁₄₀₆in which the luminance is saturated in the gamma curve 1406 and the grayscale region G₁₄₀₂ in which the luminance is saturated in the gammacurve 1402. In addition, luminance in a gray scale region in which theluminance is saturated is different from each other. That is,G₁₄₀₂>G₁₄₀₆ and a₁<a₂ are satisfied.

An advantageous effect on a displayed image due to a difference in thesize of the gray scale regions is described. Although the histogram 1404shown in FIG. 14G is similar to the histogram 1404 shown in FIG. 14D, adisplayed gray scale region is not G₁₄₀₂ but G₁₄₀₆. When FIGS. 14D and14G are compared with each other, it is apparent that the histogram 1404has a certain number of data in the gray scale region G₁₄₀₂ but thehistogram 1404 has almost no data in the gray scale region G₁₄₀₆.Therefore, it can be said that an image having a data distributionrepresented by the histogram 1404 has a lower degree of blown-outhighlights in the case where the image is displayed in accordance withthe gamma curve 1406 than the case where the image is displayed inaccordance with the gamma curve 1402.

Therefore, if the number of data included in the gray scale region G₁₄₀₂is equal to or greater than 1/10 of the total number of data in theimage displayed by the histogram 1404, a degree of blown-out highlightsin image display can be made not to be perceived by changing a gammacurve used for display from the gamma curve represented by the curve1402 to the curve represented by the curve 1406.

In this manner, the method in this document analyzes a histogram of animage and determines whether the number of data of an image included ina gray scale region in which luminance is saturated is equal to or lessthan 1/10 of the total number of data. When the number of data of theimage included in the gray scale region in which the luminance issaturated is equal to or greater than 1/10 of the total number of data,the transmittance T increases such that the graph has a gamma curvewhich is in accordance with the function T₂ (G) supplying luminancewhich is lower than that of the function T₁ (G), and the backlightluminance B_(L) decreases. In addition, the backlight luminance B_(L) ispreferably decreased by controlling the lighting ratio R. Thus, powerconsumption of a backlight can be reduced and a blanking interval can beprovided, so that motion blur can be reduced.

Note that even in the case of the gamma curve which is in accordancewith the function T₂ (G) supplying the luminance which is lower thanthat of the function T₁ (G), display with a lower degree of blown-outhighlights can be performed by not using the function T₂ (G) butseparately preparing a function supplying luminance which is lower thanthat of the function T₂ (G) in the case of an image having a histogramin which the number of data included in the gray scale region G₁₄₀₆ inwhich the luminance is saturated is equal to or greater than 1/10 of thetotal number of data (e.g., a histogram shown in FIG. 14H).

Next, a method in which peak luminance can be improved by controllingthe transmittance T and the backlight luminance B_(L) is described. Peakluminance corresponds to the highest luminance which can be displayed bya display device. When peak luminance is high, expressive power of animage is improved. For example, an image where stars twinkle in thenight sky, an image where light is reflected by a car body, or the likecan be displayed as expression which is closer to real objects.

The highest luminance can be simply increased by just increasing thebacklight luminance. However, luminance on a lower gray scale side isalso increased at the same time when the backlight luminance is justincreased, and a condition where luminance of a portion displaying blackincreases (i.e., black blurring) is caused. Thus, expressive power of animage is not improved. In order to improve expressive power of an image,it is important to increase the highest luminance without causing blackblurring. In this document, description “peak luminance is improved” maymean that the highest luminance increases without causing blackblurring.

A curve 1501 shown in FIG. 15A is a curve in which the transmittance Tof the curve 1400 decreases in each gray scale G and is represented as afunction T₃ (G). In FIG. 15A, since the backlight luminance B_(L) is notchanged, the luminance B_(L)T is lower than the luminance of the curve1400. In addition, the transmittance T in the highest gray scale is themaximum value which can be obtained by a display element.

A curve 1502 shown in FIG. 15B is a gamma curve at the time when thetransmittance T decreases as in the curve 1501 shown in FIG. 15A and thebacklight luminance B_(L) increases. At this time, a region of a grayscale where luminance of the curve 1502 is higher than the luminance ofthe curve 1400 corresponds to a gray scale region G₁₅₀₂. The highestluminance is denoted by a₃. In gray scale regions other than the grayscale region G₁₅₀₂, a shape of the curve 1502 preferably corresponds tothat of the curve 1400. Thus, even when the backlight luminance B_(L)increases, display which is similar to display when the backlightluminance B_(L) is not increased can be performed in the gray scaleregions other than the gray scale region G₁₅₀₂. Therefore, blackblurring can be suppressed.

When image display is performed by using the curve 1502 as a gammacurve, the highest luminance can be increased without causing blackblurring in a low gray scale region. That is, peak luminance can beimproved. Thus, expressive power of an image can be improved.

Note that an advantage of the method in this document is that thebacklight luminance B_(L) can be decreased by controlling the lightingratio R. Thus, a suitable blanking interval can be set, so that aflicker can be reduced and motion blur can be reduced optimally.

Here, when a curve represented by the curve 1502 shown in FIG. 15B isused as a gamma curve, an image having the large number of data includedin the gray scale region G₁₅₀₂ as in a histogram 1503 shown in FIG. 15Chas a larger effect on improvement in peak luminance. Specifically, whenthe number of data included in the gray scale region G₁₅₀₂ is equal toor greater than ⅓ of the total number of data, it is more effective.Note that even when the number of data included in the gray scale regionG₁₅₀₂ is small, a portion displayed in accordance with data included inthe gray scale region G₁₅₀₂ is further enhanced when the image is animage (e.g., an image where stars twinkle in the night sky) having ahistogram where the number of data in the low gray scale region isconsiderably large (e.g., a histogram 1504 shown in FIG. 15D), so thatit is effective to use the curve represented by the curve 1502 shown inFIG. 15B as a gamma curve. Specifically, when the whole gray scaleregions are divided equally into a low gray scale region, anintermediate gray scale region, and a high gray scale region, it isparticularly effective to use the curve represented by the curve 1502shown in FIG. 15B as a gamma curve when data of equal to or greater than½ of the total number of data is included in the low gray scale region.

Next, another method of displaying an image normally even when thebacklight luminance B_(L) decreases by controlling the transmittance Tand the backlight luminance B_(L) is described. Since the luminanceB_(L)T is the product of the transmittance T and the backlight luminanceB_(L), various gamma curves can be realized by changing thetransmittance T and the backlight luminance B_(L).

A curve 1601 shown in FIG. 16A is a curve in which the transmittance Tof the curve 1400 increases in each gray scale G and is represented as afunction T₄ (G). In FIG. 16A, since the backlight luminance B_(L) is notchanged, the luminance B_(L)T is higher than the luminance of the curve1400. In addition, although the curve 1401 shown in FIG. 14A and thecurve 1405 shown in FIG. 14E are each saturated in a certain gray scale,the curve 1601 shown in FIG. 16A is not saturated and has a gradient inthe gray scale region in which luminance is saturated in the curve 1401and the curve 1405.

A curve 1602 shown in FIG. 16B is a gamma curve at the time when thetransmittance T increases as in the curve 1601 shown in FIG. 16A and thebacklight luminance B_(L) decreases. Here, a shape of the curve 1602preferably corresponds to that of the curve 1400 in gray scale regionsother than part of a high gray scale region. At this time, a gray scaleregion where the curve 1602 and the curve 1400 do not correspond to eachother is denoted by a gray scale region G₁₆₀₂. In addition, the highestluminance of the curve 1602 is denoted by a₄. Thus, even when powerconsumption is reduced by decreasing the backlight luminance B_(L),display which is similar to display at the time when the backlightluminance B_(L) is not decreased can be performed in gray scale regionsother than the gray scale region G₁₆₀₂. Further, since a certain degreeof difference in luminance can be obtained also in display of grayscales included in the gray scale region G₁₆₀₂, blown-out highlights ofa displayed image can be suppressed.

A curve 1603 shown in FIG. 16C is a curve in which the transmittance Tof the curve 1400 increases in each of the gray scales G and isrepresented as a function T₅ (G). In FIG. 16C, since the backlightluminance B_(L) is not changed, the luminance B_(L)T is higher than theluminance of the curve 1400. In addition, since the curve 1601 shown inFIG. 16A has the gradient in part of the high gray scale region, aprimary differential function of the function T₄ (G) is discontinuous ata boundary between regions having different shapes; however, as for thecurve 1603 shown in FIG. 16C, a primary differential function of thefunction T₅ (G) is continuous at the boundary between regions havingdifferent shapes and the curve 1603 is smooth.

A curve 1604 shown in FIG. 16D is a gamma curve at the time when thetransmittance T increases as in the curve 1603 shown in FIG. 16C and thebacklight luminance B_(L) decreases. Here, a shape of the curve 1604preferably corresponds to that of the curve 1400 in gray scale regionsother than part of a high gray scale region. At this time, a gray scaleregion where the curve 1604 and the curve 1400 do not correspond to eachother is denoted by a gray scale region G₁₆₀₄. In addition, the highestluminance of the curve 1604 is denoted by a₅. Thus, even when powerconsumption is reduced by decreasing the backlight luminance B_(L),display which is similar to display at the time when the backlightluminance B_(L) is not decreased can be performed in gray scale regionsother than the gray scale region G₁₆₀₄. Further, since a certain degreeof difference in luminance can be obtained also in display of grayscales included in the gray scale region G₁₆₀₄, blown-out highlights ofa displayed image can be suppressed. Furthermore, since a boundarybetween a gray scale region where the curve 1604 and the curve 1400correspond to each other and the gray scale region where the curve 1604and the curve 1400 do not correspond to each other is smooth, there isan advantage in that a visual boarder line in a Mach band image (a falsecontour perceived by human physiology and psychology) cannot be seen.

Note that an advantage of the method in this document is that thebacklight luminance B_(L) can be decreased by controlling the lightingratio R. Thus, power consumption of a backlight can be reduced and ablanking interval can be provided, so that motion blur can be reduced.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 3

In this embodiment mode, specific examples of the control parameter P orQ described in Embodiment Mode 1 are described. In addition, in thisembodiment mode, P is used as a sign showing a control parameter.

Here, in this document, there is not a particular distinction betweenthe case where the sign showing the control parameter is P, the casewhere the sign showing the control parameter is Q, and the case wherethe sign showing the control parameter is other than P and Q. The signshowing the control parameter is just determined for convenience.Therefore, among a plurality of specific examples of the controlparameter, which are described below, any of the specific examples maybe used as the control parameter P, or any of the specific examples maybe used as the control parameter Q. In addition, the number of thecontrol parameters is not particularly limited.

First, the case is described in which the control parameter P isdetermined by numerically analyzing image data which is displayed on adisplay device.

The displayed image is divided into an object and a background byanalyzing image data which is input to the display device. Here, theobject corresponds to a portion of the image where the control parameterP is determined. In addition, the background corresponds to portionsother than the object.

FIG. 17A is a view showing a calculation method of the control parameterP when the control parameter P is determined by a distance of an objectin the case where the object moves on the screen. In FIG. 17A, a regionshown by a sign 1701 shows an object of a current frame. In addition, aregion shown by a sign 1702 shows an object of a previous frame. Thatis, the control parameter P is determined by a distance in which theobject moves when a displayed image is changed from the previous frameto the current frame. Here, ΔX in FIG. 17A shows a component in ahorizontal direction of the distance in which the object moves. ΔY inFIG. 17A shows a component in a vertical direction of the distance inwhich the object moves. A square root of the sum of a square of ΔX andΔY is the distance in which the object moves, and the control parameterP is determined by the size thereof. Here, as the distance in which theobject moves becomes larger and the object moves faster, a degree ofmotion blur increases. Therefore, as the object moves faster, thecontrol parameter P is preferably increased. This is because thelighting ratio R is controlled such that motion blur is further reducedas the control parameter P becomes larger in Embodiment Mode 1.

FIGS. 17B to 17D are views each showing the case where a shape of anobject is used as the control parameter P. An object 1711 in FIG. 17B isan object having a shape with no corner such as a circle or an oval. Anobject 1712 in FIG. 17C is an object having a relatively simple shapewith several corners such as a quadrangle or a triangle. An object 1713in FIG. 17D is an object having a complicated shape such as hiragana(Japanese syllabary characters), katakana (square phonetic Japanesesyllabary), alphabet, or Chinese character. Here, as the shape of theobject becomes complicated, a degree of motion blur increases.Therefore, as the shape of the object becomes complicated, the controlparameter P is preferably increased. This is because the lighting ratioR is controlled such that motion blur is further reduced as the controlparameter P becomes larger in Embodiment Mode 1.

FIGS. 17E to 17G are views each showing the case where the size of anobject is used as the control parameter P. An object 1721 in FIG. 17E isan object having a size of approximately 1/100 of an area of a displayportion of a display device. An object 1722 in FIG. 17F is an objecthaving a size of approximately 1/100 to approximately 1/10 of the areaof the display portion of the display device. An object 1723 in FIG. 17Gis an object having a size of approximately 1/10 or more of the area ofthe display portion of the display device. Here, as the size of theobject becomes larger, a degree of motion blur increases. Therefore, asthe size of the object becomes larger, the control parameter P ispreferably increased. This is because the lighting ratio R is controlledsuch that motion blur is further reduced as the control parameter Pbecomes larger in Embodiment Mode 1.

FIGS. 17H and 17I are views each showing the case where a position of anobject on a display portion is used as the control parameter P. Anobject 1731 in FIG. 17H is an object having a certain distance from thecenter of a display portion of a display device. An object 1732 in FIG.17I is an object located almost in the center of the display portion ofthe display device. Here, as the position of the object becomes closerto the center, a degree of motion blur increases because the object isnoticeable for a user. Therefore, as the position of the object becomescloser to the center, the control parameter P is preferably increased.This is because the lighting ratio R is controlled such that motion bluris further reduced as the control parameter P becomes larger inEmbodiment Mode 1.

FIGS. 17J to 17L are views each showing the case where density ofobjects is used as the control parameter P. A region 1741 in FIG. 17J isa group of objects in a certain range of a display portion of a displaydevice. FIG. 17J shows the case where density of the objects in theregion 1741 is low. A region 1742 in FIG. 17K is a group of objects in acertain range of the display portion of the display device. FIG. 17Kshows the case where density of the objects in the region 1742 isintermediate. A region 1743 in FIG. 17L is a group of objects in acertain range of the display portion of the display device. FIG. 17Lshows the case where density of the objects in the region 1743 is high.Here, as density of the objects becomes higher, a degree of motion blurincreases. Therefore, as density of the objects becomes higher, thecontrol parameter P is preferably increased. This is because thelighting ratio R is controlled such that motion blur is further reducedas the control parameter P becomes larger in Embodiment Mode 1.

FIGS. 18A to 18I are views each showing the case where a difference inluminance between an object and a background is used as the controlparameter P. In addition, FIGS. 18J to 18L are diagrams in whichhistograms of images shown in FIGS. 18A to 18I are compared with eachother.

FIGS. 18A to 18C are views showing images at the time when luminance ofbackgrounds 1802, 1804, and 1806 is luminance in a low gray scaleregion. FIG. 18A shows the case where luminance of an object 1801 isluminance in the low gray scale region. FIG. 18B shows the case whereluminance of an object 1803 is luminance in an intermediate gray scaleregion. FIG. 18C shows the case where luminance of an object 1805 isluminance in a high gray scale region. In addition, histograms ofrespective images are shown by a curve 1831, a curve 1832, and a curve1833 in FIG. 18J.

In each of FIGS. 18A to 18C, as the difference in luminance between theobject and the background becomes larger, a degree of motion blurincreases because a difference between the object and the backgroundstands out. That is, the degree of motion blur in the image shown inFIG. 18C is the largest and the degree of motion blur in the image shownin FIG. 18A is the smallest. The degree of motion blur in the imageshown in FIG. 18B is intermediate therebetween. When this is describedwith reference to FIG. 18J, it can be said that a degree of motion blurincreases as a difference in gray scales of the image between a portionshowing luminance distribution of the background and a portion showingluminance distribution of the object becomes larger. Therefore, as thedifference in gray scales of the image between the portion showing theluminance distribution of the background and the portion showing theluminance distribution of the object becomes larger, the controlparameter P is preferably increased. This is because the lighting ratioR is controlled such that motion blur is further reduced as the controlparameter P becomes larger in Embodiment Mode 1.

FIGS. 18D to 18F are views showing images at the time when luminance ofbackgrounds 1812, 1814, and 1816 is luminance in an intermediate grayscale region. FIG. 18D shows the case where luminance of an object 1811is luminance in the low gray scale region. FIG. 18E shows the case whereluminance of an object 1813 is luminance in an intermediate gray scaleregion. FIG. 18F shows the case where luminance of an object 1815 isluminance in a high gray scale region. In addition, histograms ofrespective images are shown by a curve 1834, a curve 1835, and a curve1836 in FIG. 18K.

In each of FIGS. 18D to 18F, as the difference in luminance between theobject and the background becomes larger, a degree of motion blurincreases because a difference between the object and the backgroundstands out. That is, the degree of motion blur in the images shown inFIGS. 18D and 18F is the largest and the degree of motion blur in theimage shown in FIG. 18E is the smallest. Note that the degree of motionblur in the images shown in FIGS. 18D and 18F is similar to the degreeof motion blur in the image shown in FIG. 18B. This is because thedifference in luminance between the object and the background in theimages shown in FIGS. 18D and 18F is similar to the difference inluminance between the object and the background in the image shown inFIG. 18B. When this is described with reference to FIG. 18K, it can besaid that a degree of motion blur increases as a difference in grayscales of the image between a portion showing luminance distribution ofthe background and a portion showing luminance distribution of theobject becomes larger. Therefore, as the difference in gray scales ofthe image between the portion showing the luminance distribution of thebackground and the portion showing the luminance distribution of theobject becomes larger, the control parameter P is preferably increased.This is because the lighting ratio R is controlled such that motion bluris further reduced as the control parameter P becomes larger inEmbodiment Mode 1.

FIGS. 18G to 18I are views showing images at the time when luminance ofbackgrounds 1822, 1824, and 1826 is luminance in a high gray scaleregion. FIG. 18G shows the case where luminance of an object 1821 isluminance in the low gray scale region. FIG. 18H shows the case whereluminance of an object 1823 is luminance in an intermediate gray scaleregion. FIG. 18I shows the case where luminance of an object 1825 isluminance in a high gray scale region. In addition, histograms ofrespective images are shown by a curve 1837, a curve 1838, and a curve1839 in FIG. 18L.

In each of FIGS. 18G to 18I, as the difference in luminance between theobject and the background becomes larger, a degree of motion blurincreases because a difference between the object and the backgroundstands out. That is, the degree of motion blur in the image shown inFIG. 18G is the largest and the degree of motion blur in the image shownin FIG. 18I is the smallest. The degree of motion blur in the imageshown in FIG. 18H is intermediate therebetween. When this is describedwith reference to FIG. 18L, it can be said that a degree of motion blurincreases as a difference in gray scales of the image between a portionshowing luminance distribution of the background and a portion showingluminance distribution of the object becomes larger. Therefore, as thedifference in gray scales of the image between the portion showing theluminance distribution of the background and the portion showing theluminance distribution of the object becomes larger, the controlparameter P is preferably increased. This is because the lighting ratioR is controlled such that motion blur is further reduced as the controlparameter P becomes larger in Embodiment Mode 1.

In this manner, a difference in luminance between an object and abackground is analyzed by a histogram, and the control parameter Pincreases (the lighting ratio R decreases) as a difference betweenluminance distribution of the object and luminance distribution of thebackground becomes larger. Therefore, motion blur can be reduced.

Note that the control parameter P can be determined by not only thedifference in luminance between the object and the background, but alsosharpness of change in luminance at a boundary between the object andthe background. That is, the control parameter P may be determined by avalue obtained by secondarily differentiating a function whichcorresponds to luminance with respect to a position in a display portionof a display device on a line including the boundary between the objectand the background. Here, as the secondary differential value at theboundary between the object and the background becomes larger, a degreeof motion blur increases because an image is an image where the boundarybetween the object and the background stands out. Therefore, as thesecondary differential value at the boundary between the object and thebackground becomes larger, the control parameter P is preferablyincreased. This is because the lighting ratio R is controlled such thatmotion blur is further reduced as the control parameter P becomes largerin Embodiment Mode 1.

Next, the case is described in which the control parameter P isdetermined by a method other than the method of numerically analyzingimage data which is displayed on a display device.

As a method of determining the control parameter P other than the methodof numerically analyzing image data which is displayed on a displaydevice, a method of collecting data on environment where a displaydevice is set can be given.

For example, a display device 1900 described in this document is set ina room as shown in FIG. 19A. The display device 1900 is set on a board1901. A temperature and humidity control device 1902 is provided on awall surface which is an upper part of the display device 1900. A window1903 is provided on a wall surface which is a left part seeing from adisplay device 1900 side. A lighting device 1904 is provided an upperpart of front seeing from the display device 1900 side. An entrance 1905is provided on a wall surface of front seeing from the display device1900 side. Particularly important items as data on environment where thedisplay device 1900 is set are heat and light.

In the environment where the display device 1900 is set, some change intemperature due to various factors always occurs. For example, when somekind of electronic and electric device is put inside the board 1901 orthe board 1901 itself is some kind of electronic and electric device,change in temperature in the display device 1900 due to heat from alower part is inevitable. In addition, when air delivered from thetemperature and humidity control device 1902 directly or indirectlyflows to the display device 1900, change in temperature in the displaydevice 1900 due to heat or cool air from an upper part is inevitable.The same can be said for the window 1903 and the entrance 1905.

When temperature of the environment where the display device 1900 is setis changed, characteristics of a display element is changed. Forexample, in the case of a liquid crystal element, response speed isquickened when temperature is high and the response speed decreases whenthe temperature is low. Therefore, as the temperature of the environmentbecomes lower, the control parameter P is preferably increased.

In this manner, the control parameter P which determines a controlcondition of the display device 1900 may be determined in accordancewith change in the temperature of the environment where the displaydevice 1900 is set. Therefore, the display device 1900 may include atemperature sensor.

In addition, light which shines on a display portion of the displaydevice 1900 greatly affects a display condition of the display device1900. As light which shines on a display portion of the display device1900, light from the lighting device 1904 or penetration of externallight from the window 1903 can be given in environment shown in FIG.19A.

When light shines on the display portion of the display device 1900,contrast of an image decreases by reflected light of the light. That is,when the contrast of the image decreases by increase in the reflectedlight, a degree of motion blur decreases. Therefore, as reflected lightby the light which shines on the display portion of the display device1900 becomes less, the control parameter P is preferably increased.

In this manner, the control parameter P which determines the controlcondition of the display device 1900 may be determined in accordancewith change in brightness of the environment where the display device1900 is set. Therefore, the display device 1900 may include a photosensor.

Next, as a method of determining the control parameter P other than themethod of numerically analyzing image data which is displayed on adisplay device, a method of determining the control parameter P bycontents displayed by a display device can be given.

A view shown in FIG. 19B shows the case where the display device 1900displays a baseball game. In addition, a view shown in FIG. 19C showsthe case where the display device 1900 displays a soccer game.

When the display device 1900 displays a baseball game, an object whichis used for determining the control parameter P is a baseball ball 1910,a bat 1911 of a batter, or the like. When the display device 1900displays a soccer game, an object which is used for determining thecontrol parameter P is a soccer ball 1920, a movement of the whole imageby a pan operation on a imaging device side, or the like. In each case,a kind of the object is extremely limited.

In addition, conditions such as speed of a movement when the object isdisplayed, the shape, the size, the position, density, the difference inluminance of the background, and the sharpness of change in theluminance at the boundary between the object and the background arehardly changed during which the contents are displayed. That is, when avalue of the control parameter P, which should be set, is determined inadvance depending on kinds of the contents, a suitable control parameterP can be determined without analyzing data on an image which isdisplayed on the display device every frame.

As kinds of the contents other than those shown in FIGS. 19B and 19C,various kinds of contents such as sports other than baseball and soccer,movies, cooking programs, news programs, variety programs, musicprograms, and animations can be given. The control parameter P can beset in advance depending on various kinds of contents.

In this manner, when a suitable control parameter P can be set inadvance depending on kinds of the contents, a suitable control parameterP can be determined without analyzing data on an image which isdisplayed on the display device every frame.

Note that as a method for determining kinds of the contents, informationfrom an electronic program guide (an EPG) may be used as well asanalyzing data on the image which is displayed on the display device.

Next, as a method of determining the control parameter P other than themethod of numerically analyzing image data which is displayed on adisplay device, a method of determining the control parameter P by ageof the user can be given.

When the control parameter P is determined by age of the user of thedisplay device, the control parameter P can be determined by setting atendency of kinds of contents displayed very often depending on age inadvance.

In addition, when the control parameter P is determined by age of theuser of the display device, luminance of a backlight can be set suitablyby age of the user of the display device in order to reduce burden oneyes of the user. At this time, luminance of the backlight may becontrolled by controlling the lighting ratio R. Thus, burden on eyes canbe reduced and motion blur can be reduced.

Further, all the methods for determining the control parameter P, whichare described in this embodiment mode, may be means which can be set bythe user of the display device.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 4

In this embodiment mode, a method for increasing response speed of adisplay element when a display element in which response to a signalinput (response speed) is low, such as a liquid crystal element and anelectrophoretic element, is used as a display element provided in adisplay device is described. In particular, a method effective for thecase where a lighting ratio R is changed is described.

There are various kinds of methods for a display element using a liquidcrystal element. A method which is most widely used is a method in whicha liquid crystal element is controlled by analog voltage, such as a TNmode, a VA mode, and an IPS mode. In these methods, response time (alsoreferred to as response speed) of a liquid crystal element is several toseveral tens of ms. One frame period in the NTSC system is 16.7 ms, andresponse time of a liquid crystal element in these modes is often longerthan one frame period. Since one of causes of motion blur is thatresponse time of a display element is longer than one frame period, theresponse time of the display element is preferably at least shorter thanone frame period. Accordingly, for a display element using a liquidcrystal element, a method is used in which voltage V_(OD) (voltageproviding desired transmittance at or around the time when predeterminedtime has passed) which is different from original voltage V_(S) (voltageproviding desired transmittance after enough time passes) is applied tothe liquid crystal element in order to reduce response time of theliquid crystal element. This method is referred to as overdrive in thisdocument. Note that the voltage V_(OD) is referred to as overdrivevoltage.

Here, in at least one of driving methods of a display device accordingto this document, a blanking interval is provided in one frame period.Accordingly, response time of a display element is preferably shorterthan an image display period τ_(a) and a blanking interval τ_(b). Notethat when a liquid crystal element or the like is used as the displayelement, response time is not always shorter than the image displayperiod τ_(a) and the blanking interval τ_(b). In this embodiment mode, amethod is described in which response time of a liquid crystal elementor the like in accordance with the length of the image display periodτ_(a) and the blanking interval τ_(b) is obtained by using overdrive.

In this document, there are several methods of providing the blankinginterval τ_(b) (methods of controlling the lighting ratio R) asdescribed above, that is, (1) the method where blanking data is directlywritten to each pixel, (2) the method where the whole backlight blinks,and (3) the method where a backlight divided into areas sequentiallyblinks. First, in (1) the method where blanking data is directly writtento each pixel, a method where response time of a liquid crystal elementor the like in accordance with the length of the image display periodτ_(a) and the blanking interval τ_(b) is obtained is described withreference to FIGS. 20A to 20C.

The length of the blanking interval τ_(b) can be changed as appropriatein order to directly write blanking data to each pixel, as shown inEmbodiment Modes 1 and 2. Further, when the length of the blankinginterval τ_(b) is changed in accordance with the control parameters Pand Q described in Embodiment Mode 3, driving in accordance with a stateof an image and an environment can be realized. For example, in the casesuch that movement of an object displayed in an image is large or whereluminance difference between a background and an object displayed in animage is large, motion blur is likely to be seen. Motion blur can bereduced by increasing the length of the blanking interval τ_(b). Inaddition, in the case such that movement of an object displayed in animage is small or where luminance difference between a background and anobject displayed in an image is small, motion blur is not likely to beseen. A flicker can be reduced by reducing the length of the blankinginterval τ_(b).

In each graph shown in FIGS. 20A to 20C, a horizontal axis representstime, and a vertical axis represents voltage and transmittance of aliquid crystal element. Voltage is shown by a solid line, andtransmittance is shown by a dashed line. Note that in this embodimentmode, the voltage refers to voltage in the case of a positive signalwhen voltage of a counter electrode is 0 V. In the case of a negativesignal, polarity of voltage is inverted. Therefore, the voltage in thegraph may be considered as an absolute value of voltage applied to theliquid crystal element. A range of time used for description is a firstframe period and a second frame period. That is, the graphs shown inFIGS. 20A to 20C show change in voltage and transmittance over time in arange of two frame periods.

A value of the voltage applied to the liquid crystal element isdescribed. Voltage V_(S1) and voltage V_(S2) are original voltages whichshould be applied in the first frame period and the second frame period,respectively. Note that the voltages V_(S1) and V_(S2) have the samevalue in all graphs in FIGS. 20A to 20C. Voltages V_(OD2001) andV_(OD2002), voltages V_(OD2011) and V_(OD2012), and voltages V_(OD2021)and V_(OD2022) are overdrive voltages in the first frame period and thesecond frame period, respectively. The overdrive voltages are preferablydifferent from each other in the graphs shown in FIGS. 20A to 20C. Notethat in a frame period before the first frame period, voltage applied tothe liquid crystal element in an image display period and voltageapplied in a blanking interval are determined as appropriate, and theyare equal, for example.

Overdrive intensity refers to difference (an absolute value) betweenoverdrive voltage and original voltage. First overdrive intensity refersto overdrive intensity in the first frame period. Second overdriveintensity refers to overdrive intensity in the second frame period.

First, with reference to FIG. 20A, a relation between voltage applied tothe liquid crystal element and transmittance in each frame period isdescribed in the case where values of the image display period τ_(a) andthe blanking interval τ_(b) are the same, that is, in the case whereτ_(a)=τ_(b)=F/2 is satisfied.

In the image display period in the first frame period, the overdrivevoltage V_(OD2001) is applied at or around the end of the image displayperiod in the first frame period so that transmittance of the liquidcrystal element becomes transmittance Ta₂₀₀₁ corresponding to theoriginal voltage V_(S1). Thus, the transmittance of the liquid crystalelement becomes the transmittance Ta₂₀₀₁ at or around the end of theimage display period in the first frame period. At this time, the firstoverdrive intensity is V₂₀₀₁, and V₂₀₀₁=V_(OD2001)−V_(S1) is satisfied.

In the blanking interval in the first frame period, it is preferablethat the transmittance of the liquid crystal element becometransmittance for providing luminance in the blanking interval, at oraround the end of the blanking interval in the first frame period at thelatest. However, because of characteristics of the liquid crystalelement, it is difficult to apply overdrive voltage in a shorter time,which is for reaching transmittance corresponding to voltage applied tothe liquid crystal element of approximately 0 V. Thus, at or around theend of the blanking interval in the first frame period, thetransmittance of the liquid crystal element is not necessary to betransmittance for providing the luminance in the blanking interval.Instead, it is preferable that transmittance Tb₂₀₀₁ at the end of theblanking interval in the first frame period can be estimated from thetransmittance Ta₂₀₀₁ at the end of the image display period in the firstframe period and the length of the blanking interval τ_(b), which can beestimated from a plurality of experiments performed in advance. When thedata is stored in a memory such as a lookup table, the data can beutilized for adjusting a value of voltage applied to the liquid crystalelement.

In the image display period in the second frame period, the overdrivevoltage V_(OD2002) is applied at or around the end of the image displayperiod in the second frame period so that the transmittance of theliquid crystal element becomes transmittance Ta₂₀₀₂ corresponding to theoriginal voltage V_(S2). Thus, the transmittance of the liquid crystalelement becomes the transmittance Ta₂₀₀₂ at or around the end of theimage display period in the second frame period. At this time, thesecond overdrive intensity is V₂₀₀₂, and V₂₀₀₂=V_(OD2002)−V_(S2) issatisfied.

The image display period in the first frame period is different from theimage display period in the second frame period in the following ways:the transmittance of the liquid crystal element is the transmittance forproviding the luminance in the blanking interval at or around the startof the image display period in the first frame period, whereas thetransmittance of the liquid crystal element is not always thetransmittance for providing the luminance in the blanking interval at oraround the start of the image display period in the second frame period.In that case, the transmittance Ta₂₀₀₂ depends on not only the voltageV_(OD2002) applied in the image display period in the second frameperiod but also the transmittance Tb₂₀₀₁ at the end of the blankinginterval in the first frame period, so that appropriate transmittancecannot be obtained.

In this case, it is extremely useful in the first frame period that thetransmittance Tb₂₀₀₁ at the end of the blanking interval in the firstframe period can be estimated from the transmittance Ta₂₀₀₁ at the endof the image display period in the first frame period and the length ofthe blanking interval τ_(b). This is because even when the transmittanceof the liquid crystal element is not the transmittance for providing theluminance in the blanking interval at or around the start of the imagedisplay period in the second frame period, the transmittance Tb₂₀₀₁ atthat time is estimated; thus, the voltage V_(OD2002) applied in theimage display period in the second frame period can be adjusted inaccordance with the level of the transmittance Tb₂₀₀₁.

In the blanking interval in the second frame period, similar to theblanking interval in the first frame period, it is preferable thattransmittance Tb₂₀₀₂ at the end of the blanking interval in the secondframe period can be estimated from the transmittance Ta₂₀₀₂ at the endof the image display period in the second frame period and the length ofthe blanking interval τ_(b). Accordingly, desired transmittance can beaccurately obtained also at the end of an image display period in aframe period next to the second frame period.

The length of the blanking interval τ_(b) can be changed as appropriatein accordance with the control parameters P and Q described inEmbodiment Mode 3. With reference to FIG. 20B, a relation betweenvoltage applied to the liquid crystal element and transmittance in eachframe period is described in the case where the image display periodτt_(a) is longer than the blanking interval τ_(b), that is, in the casewhere τ_(a)>τ_(b) is satisfied.

In the image display period in the first frame period, the overdrivevoltage V_(OD2011) is applied at or around the end of the image displayperiod in the first frame period so that the transmittance of the liquidcrystal element becomes transmittance Ta₂₀₁₁ corresponding to theoriginal voltage V_(S1). Thus, the transmittance of the liquid crystalelement becomes the transmittance Ta₂₀₁₁ at or around the end of theimage display period in the first frame period. At this time, the firstoverdrive intensity is V₂₀₁₁, and V₂₀₁₁=V_(OD2011)−V_(S1) is satisfied.

In the driving method of a display device according to this document, itis extremely useful that the first overdrive intensity V₂₀₀₁ in the casewhere τ_(a)=Σ_(b)=F/2 is satisfied shown in FIG. 20A and the firstoverdrive intensity V₂₀₁₁ in the case where τ_(a)>τ_(b) is satisfiedshown in FIG. 20B are different and V₂₀₀₁>V₂₀₁₁ is satisfied. This isbecause the image display period τ_(a) is longer in the case whereτ_(a)>τ_(b) is satisfied, so that a longer period of time can be allowedto reach desired transmittance. Accordingly, desired transmittance canbe accurately obtained by applying overdrive voltage which variesdepending on the lighting ratio R even with the same original voltageV_(S1). Note that increase in length of the image display period τ_(a)or reduction in length of the blanking interval τ_(b) is preferablydetermined in accordance with the control parameters P and Q describedin Embodiment Mode 3. This is because when it is estimated by thecontrol parameters P and Q that motion blur is not likely to be seenfrom a state of an image (e.g., the case where movement of an objectdisplayed in the image is small or the case where luminance differencebetween a background and an object displayed in the image is small) andan environment, driving by which a flicker or the like can be reduced byreducing the length of the blanking interval τ_(b) can be realized.

In the blanking interval in the first frame period, it is preferablethat the transmittance of the liquid crystal element becometransmittance for providing luminance in the blanking interval, at theend of the blanking interval in the first frame period at the latest orat the time close thereto. However, because of characteristics of theliquid crystal element, it is difficult to apply overdrive voltage in ashorter time, which is for reaching transmittance corresponding tovoltage applied to the liquid crystal element of approximately 0 V.Thus, at or around the end of the blanking interval in the first frameperiod, the transmittance of the liquid crystal element is not necessaryto be transmittance for providing the luminance in the blankinginterval. Instead, it is preferable that transmittance Tb₂₀₀₁ at the endof the blanking interval in the first frame period can be estimated fromthe transmittance Ta₂₀₁₁ at the end of the image display period in thefirst frame period and the length of the blanking interval τ_(b), whichcan be estimated from a plurality of experiments performed in advance.When the data is stored in a memory such as a lookup table, the data canbe utilized for adjusting a value of voltage applied to the liquidcrystal element.

Note that as shown in FIG. 20B, the blanking interval is further reducedin the case where τ_(a)>τ_(b) is satisfied, so that difference betweenthe transmittance Tb₂₀₁₁ at the end of the blanking interval in thefirst frame period and transmittance providing the luminance in theblanking interval is further increased. Accordingly, it is veryimportant that the transmittance Tb₂₀₀₁ at the end of the blankinginterval in the first frame period can be estimated.

In the image display period in the second frame period, the overdrivevoltage V_(OD2012) is applied at or around the end of the image displayperiod in the second frame period so that the transmittance of theliquid crystal element becomes transmittance Ta₂₀₁₂ corresponding to theoriginal voltage V_(S2). Thus, the transmittance of the liquid crystalelement becomes the transmittance Ta₂₀₁₂ at or around the end of theimage display period in the second frame period. At this time, thesecond overdrive intensity is V₂₀₁₂, and V₂₀₁₂=V_(OD2012)−V_(S2) issatisfied.

The image display period in the first frame period is different from theimage display period in the second frame period in the following ways:the transmittance of the liquid crystal element is the transmittance forproviding the luminance in the blanking interval at or around the startof the image display period in the first frame period, whereas thetransmittance of the liquid crystal element is not always thetransmittance for providing the luminance in the blanking interval at oraround the start of the image display period in the second frame period.In that case, the transmittance Ta₂₀₁₂ depends on not only the voltageV_(OD2012) applied in the image display period in the second frameperiod but also the transmittance Tb₂₀₁₁ at the end of the blankinginterval in the first frame period, so that appropriate transmittancecannot be obtained.

In this case, it is extremely useful in the first frame period that thetransmittance Tb₂₀₁₁ at the end of the blanking interval in the firstframe period can be estimated from the transmittance. Ta₂₀₁₁ at the endof the image display period in the first frame period and the length ofthe blanking interval τ_(b). This is because even when the transmittanceof the liquid crystal element is not the transmittance for providing theluminance in the blanking interval at or around the start of the imagedisplay period in the second frame period, the transmittance Tb₂₀₁₁ atthat time is estimated; thus, the voltage V_(OD2012) applied in theimage display period in the second frame period can be adjusted inaccordance with the level of the transmittance Tb₂₀₁₁.

In the driving method of a display device according to this document, itis extremely useful that the second overdrive intensity V₂₀₀₂ in thecase where τ_(a)=Σ_(b)=F/2 is satisfied shown in FIG. 20A and the secondoverdrive intensity V₂₀₁₂ in the case where τ_(a)>T_(b) is satisfiedshown in FIG. 20B are different and V₂₀₀₂>V₂₀₁₂ is satisfied. This isbecause the image display period τ_(a) is longer in the case whereτ_(a)>τ_(b) is satisfied, so that a longer period of time can be allowedto reach desired transmittance. Accordingly, desired transmittance canbe accurately obtained by applying overdrive voltage which variesdepending on the lighting ratio R even with the same original voltageV_(S2). Note that increase in length of the image display period τ_(a)or reduction in length of the blanking interval τ_(b) is preferablydetermined in accordance with the control parameters P and Q describedin Embodiment Mode 3. This is because when it is estimated by thecontrol parameters P and Q that motion blur is not likely to be seenfrom a state of an image (e.g., the case where movement of an objectdisplayed in the image is small or the case where luminance differencebetween a background and an object displayed in the image is small) andan environment, driving by which a flicker or the like can be reduced byreducing the length of the blanking interval τ_(b) can be realized.

In addition, the second overdrive intensity V₂₀₁₂ is preferably furtherreduced in the case where τ_(a)>τ_(b) is satisfied because thetransmittance Tb₂₀₁₁ at the start of the second frame period is largerthan the transmittance Tb₂₀₀₁ at the start of the second frame period inthe case where τ_(a)=τ_(b) is satisfied. That is, the second overdriveintensity V₂₀₁₂ in the case where τ_(a)>τ_(b) is satisfied is preferablysmaller than the second overdrive intensity V₂₀₀₂ in the case whereτ_(a)=τ_(b) is satisfied not only because of increase in the imagedisplay period τ_(a) but also because of increase in the transmittanceTb₂₀₁₁ at the start of the second frame period.

In the blanking interval in the second frame period, similar to theblanking interval in the first frame period, it is preferable thattransmittance Tb₂₀₁₂ at the end of the blanking interval in the secondframe period can be estimated from the transmittance Ta₂₀₁₂ at the endof the image display period in the second frame period and the length ofthe blanking interval τ_(b). Accordingly, desired transmittance can beaccurately obtained also at the end of an image display period in aframe period next to the second frame period.

Next, with reference to FIG. 20C, a relation between voltage applied tothe liquid crystal element and transmittance in each frame period isdescribed in the case where the image display period τ_(a) is shorterthan the blanking interval τ_(b), that is, in the case where τ_(a)<τ_(b)is satisfied.

In the image display period in the first frame period, the overdrivevoltage V_(OD2021) is applied at or around the end of the image displayperiod in the first frame period so that the transmittance of the liquidcrystal element becomes transmittance Ta₂₀₂₁ corresponding to theoriginal voltage V_(S1). Thus, the transmittance of the liquid crystalelement becomes the transmittance Ta₂₀₂₁ at or around the end of theimage display period in the first frame period. At this time, the firstoverdrive intensity is V₂₀₂₁; and V₂₀₂₁=V_(OD2021)−V_(S1) is satisfied.

In the driving method of a display device according to this document, itis extremely useful that the first overdrive intensity V₂₀₀₁ in the casewhere τ_(a)=τ_(b)=F/2 is satisfied shown in FIG. 20A and the firstoverdrive intensity V₂₀₂₁ in the case where τ_(a)<τ_(b) is satisfiedshown in FIG. 20C are different and V₂₀₀₁<V₂₀₂₁ is satisfied. This isbecause the image display period τ_(a) is shorter in the case whereτ_(a)<τ_(b) is satisfied, so that a shorter period of time needs to beallowed to reach desired transmittance. Accordingly, desiredtransmittance can be accurately obtained by applying overdrive voltagewhich varies depending on the lighting ratio R even with the sameoriginal voltage V_(S1). Note that reduction in length of the imagedisplay period τ_(a) or increase in length of the blanking intervalτ_(b) is preferably determined in accordance with the control parametersP and Q described in Embodiment Mode 3. This is because when it isestimated by the control parameters P and Q that motion blur is likelyto be seen from a state of an image (e.g., the case where movement of anobject displayed in the image is large or the case where luminancedifference between a background and an object displayed in the image islarge) and an environment, driving by which motion blur can be reducedby increasing the length of the blanking interval τ_(b) can be realized.

In the blanking interval in the first frame period, it is preferablethat the transmittance of the liquid crystal element becometransmittance for providing luminance in the blanking interval, at theend of the blanking interval in the first frame period at the latest orat the time close thereto. However, because of characteristics of theliquid crystal element, it is difficult to apply overdrive voltage in ashorter time, which is for reaching transmittance corresponding tovoltage applied to the liquid crystal element of approximately 0 V.Thus, at or around the end of the blanking interval in the first frameperiod, the transmittance of the liquid crystal element is not necessaryto be transmittance for providing the luminance in the blankinginterval. Instead, it is preferable that transmittance Tb₂₀₂₁ at the endof the blanking interval in the first frame period can be estimated fromthe transmittance Ta₂₀₂₁ at the end of the image display period in thefirst frame period and the length of the blanking interval τ_(b).

Note that as shown in FIG. 20C, the blanking interval is furtherincreased in the case where τ_(a)<τ_(b) is satisfied, so that differencebetween the transmittance Tb₂₀₂₁ at the end of the blanking interval inthe first frame period and transmittance providing the luminance in theblanking interval is reduced. Accordingly, the transmittance Tb₂₀₂₁ atthe end of the blanking interval in the first frame period may beestimated or the estimate may be omitted.

In the image display period in the second frame period, the overdrivevoltage V_(OD2022) is applied at or around the end of the image displayperiod in the second frame period so that the transmittance of theliquid crystal element becomes transmittance Ta₂₀₂₂ corresponding to theoriginal voltage V_(S2). Thus, the transmittance of the liquid crystalelement becomes the transmittance Ta₂₀₂₂ at or around the end of theimage display period in the second frame period. At this time, thesecond overdrive intensity is V₂₀₂₂, and V₂₀₂₂=V_(OD2022)−V_(S2) issatisfied.

The image display period in the first frame period is different from theimage display period in the second frame period in the following ways:the transmittance of the liquid crystal element is the transmittance forproviding the luminance in the blanking interval at or around the startof the image display period in the first frame period, whereas thetransmittance of the liquid crystal element is not always thetransmittance for providing the luminance in the blanking interval at oraround the start of the image display period in the second frame period.In that case, the transmittance Ta₂₀₂₂ depends on not only the voltageV_(OD2022) applied in the image display period in the second frameperiod but also the transmittance Tb₂₀₂₁ at the end of the blankinginterval in the first frame period, so that appropriate transmittancecannot be obtained.

In this case, in the first frame period, the transmittance Tb₂₀₂₁ at theend of the blanking interval in the first frame period may be estimatedfrom the transmittance Ta₂₀₂₁ at the end of the image display period inthe first frame period and the length of the blanking interval τ_(b).This is because even when the transmittance of the liquid crystalelement is not the transmittance for providing the luminance in theblanking interval at or around the start of the image display period inthe second frame period, the transmittance Tb₂₀₂₁ at that time isestimated; thus, the voltage V_(OD2022) applied in the image displayperiod in the second frame period can be adjusted in accordance with thelevel of the transmittance Tb₂₀₂₁.

In the driving method of a display device according to this document, itis extremely useful that the second overdrive intensity V₂₀₀₂ in thecase where τ_(a)=τ_(b)=F/2 is satisfied shown in FIG. 20A and the secondoverdrive intensity V₂₀₂₂ in the case where τ_(a)<τ_(b) is satisfiedshown in FIG. 20C are different and V₂₀₀₂<V₂₃₂₂ is satisfied. This isbecause the image display period τ_(a) is shorter in the case whereτ_(a)<τ_(b) is satisfied, so that a shorter period of time needs to beallowed to reach desired transmittance. Accordingly, desiredtransmittance can be accurately obtained by applying overdrive voltagewhich varies depending on the lighting ratio R even with the sameoriginal voltage V_(S2). Note that reduction in length of the imagedisplay period τ_(a) or increase in length of the blanking intervalτ_(b) is preferably determined in accordance with the control parametersP and Q described in Embodiment Mode 3. This is because when it isestimated by the control parameters P and Q that motion blur is likelyto be seen from a state of an image (e.g., the case where movement of anobject displayed in the image is large or the case where luminancedifference between a background and an object displayed in the image islarge) and an environment, driving by which motion blur can be reducedby increasing the length of the blanking interval τ_(b) can be realized.

In addition, the second overdrive intensity V₂₀₂₂ is preferably furtherincreased in the case where τ_(a)<τ_(b) is satisfied because thetransmittance Tb₂₀₂₁ at the start of the second frame period is smallerthan the transmittance Tb₂₀₀₁ at the start of the second frame period inthe case where τ_(a)=τ_(b) is satisfied. That is, the second overdriveintensity V₂₀₂₂ in the case where τ_(a)<τ_(b) is satisfied is preferablylarger than the second overdrive intensity V₂₀₀₂ in the case whereτ_(a)=τ_(b) is satisfied not only because of reduction in the imagedisplay period τ_(a) but also because of reduction in the transmittanceTb₂₀₂₁ at the start of the second frame period.

In the blanking interval in the second frame period, similar to theblanking interval in the first frame period, it is preferable thattransmittance Tb₂₀₂₂ at the end of the blanking interval in the secondframe period can be estimated from the transmittance Ta₂₀₂₂ at the endof the image display period in the second frame period and the length ofthe blanking interval τ_(b). Accordingly, desired transmittance can beaccurately obtained also at the end of an image display period in aframe period next to the second frame period.

Note that in the case where τ_(a)<τ_(b) is satisfied, difference betweenthe transmittance Tb₂₀₂₂ at the end of the blanking interval in thesecond frame period and transmittance providing the luminance in theblanking interval is smaller. Accordingly, the transmittance Tb₂₀₂₂ atthe end of the blanking interval in the second frame period may beestimated or the estimate may be omitted.

In the method where blanking data is directly written to each pixel,backlight luminance may be changed. For example, when the level of adata signal written to a pixel is the same, luminance which human eyesperceive becomes lower as the image display period τ_(a) becomes shorterand the blanking interval τ_(b) becomes longer. Accordingly, inaccordance with the length of the image display period τ_(a) and thelength of the blanking interval τ_(b) (i.e., the lighting ratio R), thebacklight luminance is reduced when the lighting ratio R is high,whereas the backlight luminance is increased when the lighting ratio Ris low. Thus, luminance which human eyes perceive can be constant.Further, the lighting ratio R preferably depends on the controlparameters P and Q described in Embodiment Mode 3. This is because thelighting ratio R can be controlled as appropriate by perceivability ofmotion blur in an image to be displayed.

Next, in (2) the method where the whole backlight blinks among themethods of controlling the lighting ratio R, a method where responsetime of a liquid crystal element is increased is described.

In (2) the method where the whole backlight blinks, a period when datawritten to a pixel is updated is referred to as one frame period. Atthis time, in the case where overdrive is used to increase responsespeed of a liquid crystal element, the overdrive voltage V_(OD) isapplied to the liquid crystal element so that the liquid crystal elementhas desired transmittance at or around the time when one frame periodpasses after voltage is applied to the liquid crystal element.

However, in (2) the method where the whole backlight blinks, timing whenvoltage is applied to the liquid crystal element in a backlight lightingperiod varies depending on a scan position. Accordingly, even when thesame overdrive voltage V_(OD) is applied to the liquid crystal element,luminance varies depending on a position of a scan line to which theliquid crystal element is connected. Accordingly, in (2) the methodwhere the whole backlight blinks, it is effective to determine theoverdrive voltage V_(OD) in consideration of this point. Further,luminance can be corrected by correcting a gray scale to be displayeddepending on a position of a scan line, other than by a method ofcontrolling the overdrive voltage V_(OD).

This is described with reference to FIGS. 21A to 21F. FIG. 21A is agraph showing timing of writing data and timing of blinking the wholebacklight on the same time axis with respect to a position of a scanline.

In the method shown in FIG. 21A, at or around the start of one frameperiod, data writing starts sequentially from a pixel connected to ascan line in the first row. Then, writing to pixels connected to allscan lines ends at or around the time when a half of one frame periodpasses. Then, the backlight is lit when writing to the pixels connectedto all the scan lines ends or at the time close thereto, and thebacklight is turned off when one frame period ends or at the time closethereto.

FIG. 21B is a graph showing change in voltage applied to the liquidcrystal element and transmittance in the pixel connected to the scanline in the first row (a position described as (B) in FIG. 21A). Notethat a time axis of the graph of FIG. 21B corresponds to that of thegraph of FIG. 21A. Voltage V_(OD2101) (original voltage V_(S2101)) isapplied in a first frame period, and the voltage V_(S2101) is applied ina second frame period.

In the first frame period, the transmittance in the graph of FIG. 21Bgradually changes from the time when data is written, and thetransmittance becomes desired transmittance when one frame period passesor at the time close thereto. At this time, the backlight lightingperiod starts before change in transmittance ends and the backlightlighting period ends when change in transmittance ends. Here, luminancewhich human eyes perceive in the first frame period depends on the areaof a portion L₂₁₀₁ shown by oblique lines in the first frame period.

In the second frame period, the transmittance in the graph of FIG. 21Bis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of a portion L₂₁₀₂ shown by oblique lines in the secondframe period.

Desired luminance for display is the same in the first frame period andthe second frame period. However, the area of the oblique line portionL₂₁₀₁ and the area of the oblique line portion L₂₁₀₂ are different fromeach other, so that luminance which human eyes perceive is different inthe first frame period and the second frame period.

In (2) the method where the whole backlight blinks, the original voltageV_(S2101) in the first frame period may be changed to correct luminancedifference between frames. That is, luminance difference between framescan be corrected by correcting gray scale data itself to be written toeach pixel. Luminance difference between frames which may cause colorshading in displaying a moving image and motion blur can be reduced bythe method according to this document.

As a method for correcting data, a method shown in FIG. 23A can be used,for example. In the method shown in FIG. 23A, the original voltage inthe first frame period is corrected from V_(S2302) to V_(S2301) in orderthat the area of oblique line regions L₂₃₀₁ and L₂₃₀₂, which representluminance in the first frame period and the second frame period, are thesame. At this time, as overdrive voltage V_(OD2301) written to eachpixel, voltage calculated from the original voltage V_(S2301) aftercorrection by using a normal method can be used. By the original voltageV_(S2301) after correction, the area of oblique line region L₂₃₀₁ andthe area of oblique line region L₂₃₀₂ are corrected to be the same. Thatis, the original voltage V_(S2301) is determined so that the area of tworegions L_(2301a) and L_(2301b) which are surrounded by an actualtransmittance curve changed by the overdrive voltage V_(OD2301) and astraight line representing transmittance in saturation when the originalvoltage V_(S2302) is applied have approximately the same area. Note thatit is preferable to correct gray scale data so that luminance of a pixelconnected to a scan line in which timing of writing is lower becomeshigher. That is, it is preferable to increase the amount of correctionof the gray scale data gradually in accordance with sequential scanningso that luminance of a pixel connected to a scan line in the last row isthe highest.

Description is made with reference to the graph of FIG. 21B again. In(2) the method where the whole backlight blinks, the overdrive voltageV_(OD2101) in the first frame period may be changed in order to correctluminance difference between frames. In general, overdrive voltage isonly for making transmittance when next writing starts in a pixel closerto desired transmittance. In the method according to this document,overdrive voltage can also be used for correcting luminance differencebetween frames. Luminance difference between frames which may causecolor shading in displaying a moving image and motion blur can bereduced by the method according to this document.

As a method for correcting overdrive voltage, a method shown in FIG. 23Ccan be used, for example. In the method shown in FIG. 23C, the overdrivevoltage is corrected to V_(OD2321) in order that the area of obliqueline regions L₂₃₂₁ and L₂₃₂₂, which represent luminance in the firstframe period and the second frame period, are the same. At this time, asthe overdrive voltage V_(OD2321), voltage obtained from a special lookuptable considering correction can be used. By the overdrive voltageV_(OD2321) after correction, the area of oblique line region L₂₃₂₁ andthe area of oblique line region L₂₃₂₂ are corrected to be the same. Thatis, the overdrive voltage V_(OD2321) is determined so that the area oftwo regions L_(2321a) and L_(2321b) which are surrounded by an actualtransmittance curve changed by the overdrive voltage V_(OD2321) and astraight line representing transmittance in saturation when originalvoltage V_(S2321) is applied have approximately the same area. Note thatit is preferable to correct overdrive voltage so that luminance of apixel connected to a scan line in which timing of writing is lowerbecomes higher. That is, it is preferable to increase the amount ofcorrection of the overdrive voltage gradually in accordance withsequential scanning so that luminance of a pixel connected to a scanline in the last row is the highest.

Next, change in voltage applied to a liquid crystal element andtransmittance in a pixel connected to the scan line near the center (aposition described as (C) in FIG. 21A) is described with reference tothe graph shown in FIG. 21C. Note that a time axis of the graph of FIG.21C corresponds to that of the graph of FIG. 21A. Voltage V_(OD2111)(original voltage V_(S2111)) is applied in a first frame period, and thevoltage V_(S2111) is applied in a second frame period

In the first frame period, the transmittance in the graph of FIG. 21Cgradually changes from the time when data is written, and thetransmittance becomes desired transmittance when one frame period passesor at the time close thereto. At this time, the backlight lightingperiod starts before change in transmittance ends and the backlightlighting period ends before change in transmittance ends. Here,luminance which human eyes perceive in the first frame period depends onthe area of a portion L₂₁₁₁ shown by oblique lines in the first frameperiod.

Here, since timing when writing starts is different depending on aposition of the scan line, it should be noted that the area of theoblique line portion L₂₁₁₁ in the first frame period is different fromthe area of an oblique line portion in the first frame period in anotherscan line. This is why luminance varies depending on a position of ascan line to which the liquid crystal element is connected even when thesame overdrive voltage V_(OD) is applied to the liquid crystal element.

Variation in luminance depending on a scan position is perceived asluminance unevenness in a display portion as it is, so that it is asignificant image defect and should be improved with priority.Accordingly, in (2) the method where the whole backlight blinks, theoriginal voltage V_(S2111) in the first frame period may be changed inorder to correct luminance difference depending on a scan position. Thatis, luminance difference depending on a scan position can be correctedby correcting gray scale data itself to be written to each pixel.

As a method for correcting data, a method shown in FIG. 23B can be used,for example. In the method shown in FIG. 23B, the original voltage inthe first frame period is corrected from V_(S2302) to V_(S2311) in orderthat the area of oblique line regions L₂₃₁₁ and L₂₃₁₂, which representluminance in the first frame period and the second frame period, are thesame and each integrated luminance of pixels connected to a differentscan line in the same frame period is the same. At this time, asoverdrive voltage V_(OD2311) written to each pixel, voltage calculatedfrom the original voltage V_(S2311) after correction by using a normalmethod can be used. By the original voltage V_(S2311) after correction,the area of oblique line region L₂₃₁₁ and the area of oblique lineregion L₂₃₁₂ are corrected to be the same. That is, the original voltageV_(S2311) is determined so that the areas of two regions L_(2311a) andL_(2311b) which are surrounded by an actual transmittance curve changedby the overdrive voltage V_(OD2311) and a straight line representingtransmittance in saturation when the original voltage V_(S2302) isapplied are approximately the same. Further, in order to preventincrease in area of the region shown by the oblique lines in the secondframe period as the overdrive voltage V_(OD2311) in the first frameperiod increases, the original voltage in the second frame period mayalso be corrected in a similar manner. At this time, corrected originalvoltage is V_(S2312), and overdrive voltage obtained from the correctedoriginal voltage V_(S2312) is V_(OD2312). In the second frame periodalso, the original voltage V_(S2312) is determined so that the areas oftwo regions L_(2311a) and L_(2311b) are approximately the same,similarly in the first frame period. Note that it is preferable tocorrect gray scale data so that luminance of a pixel connected to a scanline in which timing of writing is lower becomes higher. That is, it ispreferable to increase the amount of correction of the gray scale datagradually in accordance with sequential scanning so that luminance of apixel connected to a scan line in the last row is the highest.

Description is made with reference to the graph of FIG. 21C again. In(2) the method where the whole backlight blinks, the overdrive voltageV_(OD2111) in the first frame period may be changed in order to correctluminance difference depending on a scan position. In general, overdrivevoltage is only for making transmittance at the start of next writing ina pixel closer to desired transmittance. In the method according to thisdocument, overdrive voltage can also be used for correcting luminancedifference depending on a scan position. Accordingly, luminancedifference depending on a scan position can be corrected by correctingoverdrive voltage for a gray scale to be written to each pixel.

As a method for correcting overdrive voltage, a method shown in FIG. 23Dcan be used, for example. In the method shown in FIG. 23D, the overdrivevoltage in the first frame period is corrected to V_(OD2331) in orderthat the area of oblique line regions L₂₃₃₁ and L₂₃₃₂, which representluminance in the first frame period and the second frame period, are thesame and each integrated luminance of pixels connected to a different rscan line in the same frame period is the same. At this time, as theoverdrive voltage V_(OD2331) which is written to each pixel, voltageobtained from a special lookup table considering correction can be used.By the overdrive voltage V_(OD2331) after correction, the area ofoblique line region L₂₃₃₁ and the area of oblique line region L₂₃₃₂ arecorrected to be the same. That is, the overdrive voltage V_(OD2331) isdetermined so that the areas of two regions L_(2331a) and L_(2331b)which are surrounded by an actual transmittance curve changed by theoverdrive voltage V_(OD2331) and a straight line representingtransmittance in saturation when original voltage V_(S2331) is appliedare approximately the same. Further, in order to prevent increase inarea of the region shown by the oblique lines in the second frame periodas the overdrive voltage V_(OD2331) in the first frame period increases,the original voltage in the second frame period may also be corrected ina similar manner. At this time, corrected overdrive voltage isV_(OD2332). In the second frame period also, the overdrive voltageV_(OD2332) is determined so that the areas of two regions L_(2331a) andL_(2331b) are approximately the same, similarly in the first frameperiod. Note that it is preferable to correct gray scale data so thatluminance of a pixel connected to a scan line in which timing of writingis lower becomes higher. That is, it is preferable to increase theamount of correction of the gray scale data gradually in accordance withsequential scanning so that luminance of a pixel connected to a scanline in the last row is the highest.

Description is made with reference to the graph of FIG. 21C again. Inthe second frame period, the transmittance in the graph of FIG. 21C isalready desired transmittance before data is written. At this time, thetransmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of a portion L₂₁₁₂ shown by oblique lines in the secondframe period.

Desired luminance for display is the same in the first frame period andthe second frame period. However, the area of the oblique line portionL₂₁₁₁ and the area of the oblique line portion L₂₁₁₂ are different fromeach other, so that luminance which human eyes perceive is different inthe first frame period and the second frame period.

In (2) the method where the whole backlight blinks, the original voltageV_(S2111) in the first frame period may be changed to correct luminancedifference between frames. That is, luminance difference between framescan be corrected by correcting gray scale data itself to be written toeach pixel. Luminance difference between frames which may cause colorshading in displaying a moving image and motion blur can be reduced bythe method according to this document. As a method for correcting data,the method shown in FIG. 23B can be used.

In addition, in (2) the method where the whole backlight blinks, theoverdrive voltage V_(OD2111) in the first frame period may be changed inorder to correct luminance difference between frames. In general,overdrive voltage is only for making transmittance when next writingstarts in a pixel closer to desired transmittance. In the methodaccording to this document, overdrive voltage can also be used forcorrecting luminance difference between frames. Luminance differencebetween frames which may cause color shading in displaying a movingimage and motion blur can be reduced by the method according to thisdocument. As a method for correcting overdrive voltage, the method shownin FIG. 23D can be used.

Next, change in voltage applied to a liquid crystal element andtransmittance in a pixel connected to the scan line at the bottom (aposition described as (D) in FIG. 21A) is described with reference to agraph shown in FIG. 21D. Note that a time axis of the graph of FIG. 21Dcorresponds to that of the graph of FIG. 21A. Voltage V_(OD2121)(original voltage V_(S2121)) is applied in a first frame period, and thevoltage V_(S2121) is applied in a second frame period.

In the first frame period, the transmittance in the graph of FIG. 21Dgradually changes from the time when data is written, and thetransmittance becomes desired transmittance when one frame period passesor at the time close thereto. At this time, the backlight lightingperiod starts when change in transmittance starts and the backlightlighting period ends long before change in transmittance ends. Luminancewhich human eyes perceive in the first frame period depends on the areaof a portion L₂₁₂₁ shown by oblique lines in the first frame period.

Here, since timing when writing starts is different depending on aposition of the scan line, it should be noted that the area of theoblique line portion L₂₁₁₁ in the first frame period is different fromthe area of an oblique line portion in another scan line. This is whyluminance varies depending on a position of a scan line to which theliquid crystal element is connected even when the same overdrive voltageV_(OD) is applied to the liquid crystal element.

Variation in luminance depending on a scan position is perceived asluminance unevenness in a display portion as it is, so that it is asignificant image defect and should be improved with priority.Therefore, in (2) the method where the whole backlight blinks, theoriginal voltage V_(S2121) in the first frame period may be changed inorder to correct luminance difference depending on a scan position. Thatis, luminance difference depending on a scan position can be correctedby correcting gray scale data itself to be written to each pixel. As amethod for correcting data, the method shown in FIG. 23B can be used.

In addition, in (2) the method where the whole backlight blinks, theoverdrive voltage V_(OD2121) in the first frame period may be changed inorder to correct luminance difference depending on a scan position. Ingeneral, overdrive voltage is only for making transmittance when nextwriting starts in a pixel closer to desired transmittance. In the methodaccording to this document, overdrive voltage can also be used forcorrecting luminance difference depending on a scan position.Accordingly, luminance difference depending on a scan position can becorrected by correcting overdrive voltage for a gray scale to be writtento each pixel. As a method for correcting overdrive voltage, the methodshown in FIG. 23D can be used.

Luminance difference depending on a scan position increases as positionsof scan lines are distant from each other. Accordingly, both in a methodof changing original voltage V_(S) and in a method of changing overdrivevoltage V_(OD), it is effective to increase the amount of change involtage as the positions of scan lines are distant from each other.

In the second frame period, the transmittance in the graph of FIG. 21Dis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of a portion L₂₁₂₂ shown by oblique lines in the secondframe period.

Desired luminance for display is the same in the first frame period andthe second frame period. However, the area of the oblique line portionL₂₁₂₁ and the area of the oblique line portion L₂₁₂₂ are different fromeach other, so that luminance which human eyes perceive is different inthe first frame period and the second frame period.

In (2) the method where the whole backlight blinks, the original voltageV_(S2121) in the first frame period may be changed to correct luminancedifference between frames. That is, luminance difference between framescan be corrected by correcting gray scale data itself to be written toeach pixel. Luminance difference between frames which may cause colorshading in displaying a moving image and motion blur can be reduced bythe method according to this document. As a method for correcting data,the method shown in FIG. 23B can be used.

In addition, in (2) the method where the whole backlight blinks, theoverdrive voltage V_(OD2121) in the first frame period may be changed inorder to correct luminance difference between frames. In general,overdrive voltage is only for making transmittance when next writingstarts in a pixel closer to desired transmittance. In the methodaccording to this document, overdrive voltage can also be used forcorrecting luminance difference between frames. Luminance differencebetween frames which may cause color shading in displaying a movingimage and motion blur can be reduced by the method according to thisdocument. As a method for correcting overdrive voltage, the method shownin FIG. 23D can be used.

Luminance difference depending on a scan position increases as positionsof scan lines are distant from each other. Accordingly, both in a methodof changing the original voltage V_(S) and in a method of changing theoverdrive voltage V_(OD), it is effective to increase the amount ofchange in voltage as the positions of scan lines are distant from eachother.

Note that the method of changing the original voltage V_(S) can berealized by the flow of data processing shown in FIG. 21E. First, a grayscale of data input is corrected by a gray scale correction portionwhich corrects a gray scale depending on a scan position. Thereafter,the corrected data is output to a pixel as the overdrive voltage V_(OD)by a lookup table (ODLUT) which performs normal overdrive.

Note that the method of changing the overdrive voltage V_(OD) can berealised by the flow of data processing shown in FIG. 21F. That is, datainput is processed by a special lookup table (ODLUT), which can alsocorrect a gray scale depending on a scan position at the same time, andthereafter, is output to a pixel as the overdrive voltage V_(OD).

Next, in (3) the method where a backlight divided into areassequentially blinks among the methods of controlling the lighting ratioR, a method where response time of a liquid crystal element or the likeis increased is described. Note that areas of a backlight in thisembodiment mode may be one-dimensionally or two-dimensionally divided.When a backlight is one-dimensionally divided, a linear light sourcesuch as a cold cathode fluorescent lamp (CCFL) or a hot cathodefluorescent lamp (HSFL) can be used, and the backlight can be arrangedin parallel or perpendicular to a scan line. When a backlight istwo-dimensionally divided, a point light source such as an LED or asheet light source such as EL can be used, and the light source can bearranged in matrix, honeycomb arrangement, Bayer arrangement, or thelike. Further, a structure may be employed in which light sources forrespective colors such as RGB are provided and the backlight can becontrolled for each color.

In the (3) the method where a backlight divided into areas sequentiallyblinks, a period when data written to a pixel is updated is referred toas one frame period. At this time, in the case where overdrive is usedto increase response speed of a liquid crystal element, the overdrivevoltage V_(OD) is applied to the liquid crystal element so that theliquid crystal element has desired transmittance at or around the timewhen one frame period passes after voltage is applied to the liquidcrystal element.

However, in (3) the method where a backlight divided into areassequentially blinks, timing when voltage is applied to the liquidcrystal element in a backlight lighting period in the areas variesdepending on a scan position. Accordingly, even when the same overdrivevoltage V_(OD) is applied to the liquid crystal element, luminancevaries depending on a position of a scan line to which the liquidcrystal element is connected. Accordingly, in (3) the method where abacklight divided into areas sequentially blinks, it is effective todetermine the overdrive voltage V_(OD) in consideration of this point.Further, luminance can be corrected by correcting a gray scale to bedisplayed depending on a position of a scan line, other than by themethod of controlling the overdrive voltage V_(OD).

Large difference between (3) the method where a backlight divided intoareas sequentially blinks and (2) the method where the whole backlightblinks is whether there are a plurality of areas with differentluminance in a display portion. That is, in (3) the method where abacklight divided into areas sequentially blinks, pixels with differentluminance are adjacent to each other at a boundary between a certainarea and an area adjacent thereto. Thus, luminance difference in thedisplay portion is extremely easily perceived. That is, luminancedifference depending on a scan position in (3) the method where abacklight divided into areas sequentially blinks causes more seriousimage quality degradation than luminance difference depending on a scanposition in (2) the method where the whole backlight blinks.Accordingly, the method according to this document in (3) the methodwhere a backlight divided into areas sequentially blinks is veryeffective in improving image quality.

This is described with reference to FIGS. 22A to 22D. FIG. 22A is agraph showing timing of writing data with respect to a position of ascan line and timing of sequentially blinking a backlight divided intoareas on the same time axis.

In a method shown in FIG. 22A, at or around the start of one frameperiod, data writing starts sequentially from a pixel connected to ascan line in the first row. Then, the top area of the backlight is litat or around the time when a half of one frame period passes.Thereafter, the backlight in each area sequentially starts lightingwhile the other pixels are sequentially scanned and data is written tothe pixels. Then, the top area of the backlight is turned off when oneframe period ends or at the time close thereto. After that, thebacklight in each area is sequentially turned off while writing andscanning of next frame start and data is written to pixels from the top.

FIG. 22B is a graph showing change in voltage applied to the liquidcrystal element and transmittance in the pixel connected to the scanline in the first row (a position described as (B) in FIG. 22A). Notethat a time axis of the graph of FIG. 22B corresponds to that of thegraph of FIG. 22A. Voltage V_(OD2201) (original voltage V_(S2201)) isapplied in a first frame period, and the voltage V_(S2201) is applied ina second frame period.

In the first frame period, the transmittance in the graph of FIG. 22Bgradually changes from the time when data is written, and thetransmittance becomes desired transmittance when one frame period passesor at the time close thereto. At this time, the backlight lightingperiod starts before change in transmittance ends and the backlightlighting period ends when change in transmittance ends. Here, luminancewhich human eyes perceive in the first frame period depends on the areaof a portion L₂₂₀₁ shown by oblique lines in the first frame period.

In the second frame period, the transmittance in the graph of FIG. 22Bis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of a portion L₂₂₀₂ shown by oblique lines in the secondframe period.

Desired luminance for display is the same in the first frame period andthe second frame period. However, the area of the oblique line portionL₂₂₀₁ and the area of the oblique line portion L₂₂₀₂ are different fromeach other, so that luminance which human eyes perceive is different inthe first frame period and the second frame period.

In (3) the method where a backlight divided into areas sequentiallyblinks, the original voltage V_(S2201) in the first frame period may bechanged to correct luminance difference between frames. That is,luminance difference between frames can be corrected by correcting grayscale data itself to be written to each pixel. Luminance differencebetween frames which may cause color shading in displaying a movingimage and motion blur can be reduced by the method according to thisdocument. As a method for correcting data, the method shown in FIG. 23Acan be used.

In addition, in (3) the method where a backlight divided into areassequentially blinks, the overdrive voltage V_(OD2201) in the first frameperiod may be changed in order to correct luminance difference betweenframes. In general, overdrive voltage is only for making transmittancewhen next writing starts in a pixel closer to desired transmittance. Inthe method according to this document, overdrive voltage can also beused for correcting luminance difference between frames. Luminancedifference between frames which may cause color shading in displaying amoving image and motion blur can be reduced by the method according tothis document. As a method for correcting overdrive voltage, the methodshown in FIG. 23C can be used.

Next, change in voltage applied to a liquid crystal element andtransmittance in a pixel connected to the scan line at the bottom (aposition described as (C) in FIG. 22A) among pixels belonging to the toparea of the backlight is described with reference to a graph shown inFIG. 22C. Note that a time axis of the graph of FIG. 22C corresponds tothat of the graph of FIG. 22A: Voltage V_(OD2211) (original voltageV_(S2211)) is applied in a first frame period, and the voltage V_(S2211)is applied in a second frame period.

In the first frame period, the transmittance in the graph of FIG. 22Cgradually changes from the time when data is written, and thetransmittance becomes desired transmittance when one frame period passesor at the time close thereto. At this time, the backlight lightingperiod starts before change in transmittance ends and the backlightlighting period ends before change in transmittance ends. Further,luminance which human eyes perceive in the first frame period depends onthe area of a portion L₂₂₁₁ shown by oblique lines in the first frameperiod.

Here, since timing when writing starts is different depending on aposition of the scan line, it should be noted that the area of theoblique line portion L₂₂₁₁ in the first frame period is different fromthe area of an oblique line portion in another scan line belonging tothe same area. This is why luminance varies depending on a position of ascan line to which the liquid crystal element is connected even when thesame overdrive voltage V_(OD) is applied to the liquid crystal element.

Variation in luminance depending on a scan position is perceived asluminance unevenness in a display portion as it is, so that it is asignificant image defect and should be improved with priority.Accordingly, in (3) the method where a backlight divided into areassequentially blinks, the original voltage V_(S2211) in the first frameperiod may be changed in order to correct luminance difference dependingon a scan position. That is, luminance difference depending on a scanposition can be corrected by correcting gray scale data itself to bewritten to each pixel. As a method for correcting data, the method shownin FIG. 23B can be used.

In addition, in (3) the method where a backlight divided into areassequentially blinks, the overdrive voltage V_(OD2211) in the first frameperiod may be changed in order to correct luminance difference dependingon a scan position. In general, overdrive voltage is only for makingtransmittance when next writing starts in a pixel closer to desiredtransmittance. In the method according to this document, overdrivevoltage can also be used for correcting luminance difference dependingon a scan position. Accordingly, luminance difference depending on ascan position can be corrected by correcting gray scale data itself tobe written to each pixel. As a method for correcting overdrive voltage,the method shown in FIG. 23D can be used.

In the second frame period, the transmittance in the graph of FIG. 22Cis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of a portion L₂₂₁₂ shown by oblique lines in the secondframe period.

Desired luminance for display is the same in the first frame period andthe second frame period. However, the area of the oblique line portionL₂₂₁₁ and the area of the oblique line portion L₂₂₁₂ are different fromeach other, so that luminance which human eyes perceive is different inthe first frame period and the second frame period.

In (3) the method where a backlight divided into areas sequentiallyblinks, the original voltage V_(S2211) in the first frame period may bechanged to correct luminance difference between frames. That is,luminance difference between frames can be corrected by correcting grayscale data itself to be written to each pixel. Luminance differencebetween frames which may cause color shading in displaying a movingimage and motion blur can be reduced by the method according to thisdocument. As a method for correcting data, the method shown in FIG. 23Bcan be used.

In addition, in (3) the method where a backlight divided into areassequentially blinks, the overdrive voltage V_(OD2211) in the first frameperiod may be changed in order to correct luminance difference betweenframes. In general, overdrive voltage is only for making transmittancewhen next writing starts in a pixel closer to desired transmittance. Inthe method according to this document, overdrive voltage can also beused for correcting luminance difference between frames. Luminancedifference between frames which may cause color shading in displaying amoving image and motion blur can be reduced by the method according tothis document. As a method for correcting overdrive voltage, the methodshown in FIG. 23D can be used.

Next, change in voltage applied to a liquid crystal element andtransmittance in a pixel connected to the scan line in the first row (aposition described as (D) in FIG. 22A) among pixels belonging to thesecond top area of the backlight is described with reference to a graphshown in FIG. 22D. Note that a time axis of the graph of FIG. 22Dcorresponds to that of the graph of FIG. 22A. Voltage V_(OD2221)(original voltage V_(S2221)) is applied in a first frame period, and thevoltage V_(S2221) is applied in a second frame period.

In the first frame period, the transmittance in the graph of FIG. 22Dgradually changes from the time when data is written, and thetransmittance becomes desired transmittance when one frame period passesor at the time close thereto. At this time, the backlight lightingperiod starts before change in transmittance ends and the backlightlighting period ends when change in transmittance ends. Here, luminancewhich human eyes perceive in the first frame period depends on the areaof a portion L₂₂₂₁ shown by oblique lines in the first frame period.

In the second frame period, the transmittance in the graph of FIG. 22Dis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of a portion L₂₂₂₂ shown by oblique lines in the secondframe period.

Desired luminance for display is the same in the first frame period andthe second frame period. However, the area of the oblique line portionL₂₂₂₁ and the area of the oblique line portion L₂₂₂₂ are different fromeach other, so that luminance which human eyes perceive is different inthe first frame period and the second frame period.

In (3) the method where a backlight divided into areas sequentiallyblinks, the original voltage V_(S2221) in the first frame period may bechanged to correct luminance difference between frames. That is,luminance difference between frames can be corrected by correcting grayscale data itself to be written to each pixel. Luminance differencebetween frames which may cause color shading in displaying a movingimage and motion blur can be reduced by the method according to thisdocument. As a method for correcting data, the method shown in FIG. 23Acan be used.

In addition, in (3) the method where a backlight divided into areassequentially blinks, the overdrive voltage V_(OD2221) in the first frameperiod may be changed in order to correct luminance difference betweenframes. In general, overdrive voltage is only for making transmittancewhen next writing starts in a pixel closer to desired transmittance. Inthe method according to this document, overdrive voltage can also beused for correcting luminance difference between frames. Luminancedifference between frames which may cause color shading in displaying amoving image and motion blur can be reduced by the method according tothis document. As a method for correcting overdrive voltage, the methodshown in FIG. 23C can be used.

As described above, in the pixel shown in FIG. 22B, which is connectedto the scan line in the first row (the position described as (B) in FIG.22A) in the top area, and the pixel shown in FIG. 22D, which isconnected to the scan line in the first row (the position described as(D) in FIG. 22A) in the second top area, timing when voltage is writtenis different; however, the length of the time from when voltage iswritten to when a backlight lighting period starts is the same.Accordingly, the areas of both of the oblique line portions (L₂₂₀₁ andL₂₂₂₁), which represent integrated luminance, are equal, so thatluminance of both pixels which human eyes perceive is equal.

That is, it can be said that luminance which human eyes perceive isdetermined by the time from when voltage is written to when a backlightlighting period starts. The luminance which human eyes perceive isincreased as time from when voltage is written to when a backlightlighting period starts is longer, whereas the luminance which human eyesperceive is decreased as the time from when voltage is written to when abacklight lighting period starts is shorter.

Here, in the example of FIGS. 21A to 21D, which describe (2) the methodwhere the whole backlight blinks, time from when voltage is written towhen a backlight lighting period starts in the scan line in the firstrow is a half of one frame period, whereas time from when voltage iswritten to when a backlight lighting period starts in the scan line atthe bottom is approximately 0. That is, in the example of FIGS. 21A to21D, which describe (2) the method where the whole backlight blinks, themaximum length of the time between when voltage is written and when abacklight lighting period starts is a half of one frame period.

On the other hand, in the example of FIGS. 22A to 22D, which describe(3) the method where a backlight divided into areas sequentially blinks,the maximum length of the time from when voltage is written to when abacklight lighting period starts is a half of one frame period, which isthe same as the example of FIGS. 21A to 21D, which describe (2) themethod where the whole backlight blinks. Meanwhile, even when the timefrom when voltage is written to when a backlight lighting period startsis the shortest (in the scan line at the bottom in each area), it doesnot become 0. Accordingly, in the example of FIGS. 22A to 22D, whichdescribe (3) the method where a backlight divided into areassequentially blinks, the maximum value of difference between whenvoltage is written and when a backlight lighting period starts is lessthan a half of one frame period.

Accordingly, when the lighting ratio R is the same in (2) the methodwhere the whole backlight blinks and (3) the method where a backlightdivided into areas sequentially blinks, (3) the method where a backlightdivided into areas sequentially blinks has a smaller maximum value ofluminance difference depending on a scan position.

However, as a factor in deciding image quality of a display device, notonly the maximum value of luminance difference depending on a scanposition but also a distribution of luminance difference is important.In the example of FIGS. 21A to 21F, which describe (2) the method wherethe whole backlight blinks, the maximum value of luminance differencedepending on a scan position is large, and a distribution of luminancedifference is gradual. Thus, luminance difference gently appears in thewhole image. For example, when display is performed with uniformluminance in all of pixels and after that, the same amount of luminanceis changed in all of the pixels all at once, luminance difference withgradation from an upper side to a lower side of a display portion isobserved in a transient state.

On the other hand, in the example of FIG. 22A to 22D, which describe (3)the method where a backlight divided into areas sequentially blinks, themaximum value of luminance difference depending on a scan position issmall, and a distribution of luminance difference is sharp at a boundarybetween different areas. Further, a distribution of luminance differencewithin each area is gradual. For example, when display is performed withuniform luminance in all of pixels and after that, the same amount ofluminance is changed in all of the pixels all at once, luminancedifference with gradation from an upper side to a lower side of eacharea appears in a transient state. The luminance difference withgradation is the same in each area. Accordingly, sharp luminancedifference appears at a boundary of each area. The sharp luminancedifference can be extremely easily perceived as compared with the casewhere luminance difference with gradation appears in the whole displayportion, and thus causes significant reduction in image quality.

By the method according to this document, a problem of reduction inimage quality in (3) the method where a backlight divided into areassequentially blinks can be reduced. The original voltage V_(S) may bechanged in order to correct luminance difference depending on a scanposition. That is, luminance difference depending on a scan position canbe corrected by correcting gray scale data itself to be written to eachpixel. In particular, the amount of correction of original voltage in apixel to which data is written at the end of each area is made to be thelargest in the area to which the pixel belongs, so that sharp luminancedifference at a boundary of areas can be corrected.

In addition, the overdrive voltage V_(OD) may be changed in order tocorrect luminance difference depending on a scan position. In general,overdrive voltage is only for making transmittance when next writingstarts in a pixel closer to desired transmittance. In the methodaccording to this document, overdrive voltage can also be used forcorrecting luminance difference depending on a scan position.Accordingly, luminance difference depending on a scan position can becorrected by correcting gray scale data itself to be written to eachpixel. In particular, the amount of correction of overdrive voltage in apixel to which data is written at the end of each area is made to be thelargest in the area to which the pixel belongs, so that sharp luminancedifference at a boundary of areas can be corrected.

Next, in a method of changing the lighting ratio R which is one ofmethods of controlling a display device according to this document, amethod of controlling a display device in frame periods before and afterthe lighting ratio R is changed is described in detail. Here, asdescribed in Embodiment Modes 1 and 2, changing the lighting ratio Rrefers to changing the length of the blanking interval τ_(b) asappropriate. Further, driving in accordance with a state of an image andan environment can be realized by changing the length of the blankinginterval τ_(b) in accordance with the control parameters P and Qdescribed in Embodiment Mode 3. For example, in the case such thatmovement of an object displayed in an image is large or where luminancedifference between a background and an object displayed in an image islarge, motion blur is likely to be seen. Motion blur can be reduced byincreasing the length of the blanking interval τ_(b). In addition, inthe case such that movement of an object displayed in an image is smallor where luminance difference between a background and an objectdisplayed in an image is small, motion blur is not likely to be seen.Accordingly, a flicker can be reduced by reducing the length of theblanking interval τ_(b). Note that here described is a purpose ofpreventing luminance which human eyes perceive from being changed inframe periods before and after the lighting ratio R is changed, evenwhen the lighting ratio R is changed.

Methods for preventing luminance which human eyes perceive from beingchanged in frame periods before and after the lighting ratio R ischanged are broadly classified into two methods: a method where voltagewritten to a pixel is controlled under a condition that backlightluminance is constant when a backlight is lit; and a method wherebacklight luminance is changed.

In each method, a controlling method of a display device is differentdepending on a method for providing the blanking interval τ_(b) (amethod of controlling the lighting ratio R). Accordingly, in thisdocument, the case where a method of controlling the lighting ratio R isdifferent in each method is also individually described in detail.

Note that as the method for providing the blanking interval τ_(b) (themethod of controlling the lighting ratio R), (1) a method where blankingdata is directly written to each pixel, (2) a method where the wholebacklight blinks, (3) a method where a backlight divided into areassequentially blinks, and a combination of these methods can be used.

First, the case of using (1) the method where blanking data is directlywritten to each pixel among the methods where voltage written to a pixelis controlled under a condition that backlight luminance is constantwhen a backlight is lit is described with reference to FIGS. 24A and24B.

FIG. 24A is a graph showing timing of writing data and timing of writingblanking data on the same time axis with respect to a position of a scanline when the lighting ratio R is different in the first frame periodand the second frame period. Here, for explanation, an image displayperiod and a blanking interval in the first frame period are denoted byτ_(a2401) and τ_(b2401), an image display period and a blanking intervalin and the second frame period are denoted by τ_(a2402) and τ_(b2402).

FIG. 24B is a graph showing original voltages V_(S2401) and V_(S2402)and overdrive voltages V_(OD2401) and V_(OD2402) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the image display period and the blankinginterval in the first frame period and the image display period and theblanking interval in the second frame period are similar to those inFIG. 24A. Each area of oblique line regions L₂₄₀₁ and L₂₄₀₂ representsthe level of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₄₀₁ and voltage V₂₄₀₂ is overdrive intensity in theimage display periods in the first frame period and the second frameperiod, and V₂₄₀₁=V_(OD2401)−V_(S2401) and V₂₄₀₂=V_(OD2402)−V_(S2402)are satisfied.

In the case of using (1) the method where blanking data is directlywritten to each pixel among the methods where voltage written to a pixelis controlled under a condition that backlight luminance when thebacklight is lit is constant before and after the lighting ratio R ischanged, driving can be realized by changing timing of writing blankingdata in the first frame period and the second frame period, as shown inFIG. 24A. In addition, a relation between the voltage written to eachpixel at this time and transmittance can be understood with reference toFIG. 24B.

When the overdrive voltage V_(OD2401) is applied to each pixel at oraround the start of the image display period in the first frame period,transmittance of a display element becomes transmittance correspondingto the original voltage V_(S2401) at the time when the image displayperiod in the first frame period ends or at the time close thereto.Thereafter, blanking writing is performed. Thus, integrated luminance inthe first frame period is represented by the area of the oblique lineregion L₂₄₀₁.

Then, when the overdrive voltage V_(OD2402) is applied to each pixel ator around the start of the image display period in the second frameperiod, the transmittance of the display element becomes transmittancecorresponding to the original voltage V_(S2402) at the time when theimage display period in the second frame period ends or at the timeclose thereto. Thereafter, blanking writing is performed. Thus,integrated luminance in the second frame period is represented by thearea of the oblique line region L₂₄₀₂.

At this time, it is important that values of the voltages applied to apixel vary in the first frame period and the second frame period. Thatis, in the case where the lighting ratio R is changed under a conditionthat backlight luminance is constant when a backlight is lit, it ispreferable to write different voltage in the first frame period and thesecond frame period, not the same voltage, if luminance of the pixelwhich human eyes perceive is not desired to be changed.

Accordingly, in one of methods according to this document, the originalvoltage and the overdrive voltage are changed in accordance with thelighting ratio R in order that the area of the oblique line region L₂₄₀₁in the first frame period and the area of the oblique line region L₂₄₀₂in the second frame period are approximately the same. Specifically, itis preferable to reduce the original voltage and the overdrive voltageas the lighting ratio R increases. In addition, in one of the methodsaccording to this document, the overdrive intensity V₂₄₀₁ in the firstframe period and the overdrive intensity V₂₄₀₂ in the second frameperiod may be changed in accordance with the lighting ratio R.Specifically, it is preferable to reduce the overdrive intensity as thelighting ratio R increases. This is because increase in the lightingratio R means increase in length of the image display period τ_(a), andincrease in length of the image display period τ_(a) can be allowed tohave a longer period of time for reaching intended transmittance of aliquid crystal element. Moreover, when the length of the image displayperiod τ_(a) is increased, intended transmittance of a liquid crystalelement itself can be reduced, so that the original voltage V_(S) isreduced, and further, the overdrive intensity can be reduced.

By driving a display device in such a manner, backlight luminance can beconstant even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit for driving a backlight issimplified, so that manufacturing cost can be reduced. Further,luminance unevenness and a flicker in displaying an image can bereduced. Moreover, provision of the blanking interval τ_(b) can reducemotion blur, and image quality of a moving image can be improved.

Next, the case of using (2) the method where the whole backlight blinksamong the methods where voltage written to a pixel is controlled under acondition that backlight luminance is constant when the backlight is litis described with reference to FIGS. 25A to 25C.

FIG. 25A is a graph showing timing of writing data and timing ofblinking a backlight on the same time axis with respect to a position ofa scan line when the lighting ratio R is different in the first frameperiod and the second frame period. Here, for explanation, a backlightlighting period in the first frame period is denoted by τ_(a2501), and abacklight lighting period in the second frame period is denoted byτ_(a2502).

FIG. 25B is a graph showing original voltages V_(S2501) and V_(S2502)and overdrive voltages V_(OD2501) and V_(OD2502) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.25A. Each area of oblique line regions L₂₅₀₁ and L₂₅₀₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₅₀₁ and voltage V₂₅₀₂ is overdrive intensity in thefirst frame period and the second frame period, andV₂₅₀₁=V_(OD2501)−V_(S2501) and V₂₅₀₂=V_(OD2502)−V_(S2502) are satisfied.

In the case of using (2) the method where the whole backlight blinksamong the methods where voltage written to a pixel is controlled under acondition that backlight luminance is constant when the backlight is litbefore and after the lighting ratio R is changed, driving can berealized by changing the length and timing of the backlight lightingperiod in the first frame period and the second frame period, as shownin FIG. 25A. In addition, a relation between the voltage written to eachpixel at this time and transmittance can be understood with reference toFIG. 25B.

When the overdrive voltage V_(OD2501) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S2501) at the time when the next data is written by data writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is provided in all pixels allat once. Thus, integrated luminance in the first frame period isrepresented by the area of the oblique line region L₂₅₀₁, which issurrounded by the backlight lighting period and the transmittance.

Then, when the overdrive voltage V_(OD2502) is written to the pixel bydata writing scanning in the second frame period, the transmittance ofthe display element becomes transmittance corresponding to the originalvoltage V_(S2502) at the time when the next data is written by datawriting scanning in the next frame period or at the time close thereto.In that period, a backlight lighting period is provided in all thepixels all at once. Thus, integrated luminance in the second frameperiod is represented by the area of the oblique line region L₂₅₀₂,which is surrounded by the backlight lighting period and thetransmittance.

At this time, it is important that values of the voltages applied to apixel vary in the first frame period and the second frame period. Thatis, in the case where the lighting ratio R is changed under a conditionthat backlight luminance is constant when a backlight is lit, it ispreferable to write different voltage in the first frame period and thesecond frame period, not the same voltage, if luminance of the pixelwhich human eyes perceive is not desired to be changed.

Accordingly, in one of methods according to this document, the originalvoltage and the overdrive voltage are changed in accordance with thelighting ratio R in order that the area of the oblique line region L₂₅₀₁in the first frame period and the area of the oblique line region L₂₅₀₂in the second frame period are approximately the same. Specifically, itis preferable to reduce the original voltage and the overdrive voltageas the lighting ratio R increases. In addition, in one of the methodsaccording to this document, the overdrive intensity V₂₅₀₁ in the firstframe period and the overdrive intensity V₂₅₀₂ in the second frameperiod may be changed in accordance with the lighting ratio R.Specifically, it is preferable to reduce the overdrive intensity as thelighting ratio R increases. This is because increase in the lightingratio R means increase in length of the image display period τ_(a), andincrease in length of the image display period τ_(a) can be allowed tohave a longer period of time for reaching intended transmittance of aliquid crystal element. Moreover, when the length of the image displayperiod τ_(a) is increased, intended transmittance of a liquid crystalelement itself can be reduced, so that the original voltage V_(S) isreduced, and further, the overdrive intensity can be reduced.

By driving a display device in such a manner, backlight luminance can beconstant even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit for driving a backlight issimplified, so that manufacturing cost can be reduced. Further,luminance unevenness and a flicker in displaying an image can bereduced. Moreover, provision of the blanking interval τ_(b) can reducemotion blur, and image quality of a moving image can be improved.

FIG. 25C is a graph showing original voltages V_(S2511) and V_(S2512)and overdrive voltages V_(OD2511) and V_(OD2512) written to each pixel,and transmittance with respect to each voltage on the same time axis ina pixel connected to a scan line different from that shown in FIG. 25Bwhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.25A. Each area of oblique line regions L₂₅₁₁ and L₂₅₁₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₅₁₁ and voltage V₂₅₁₂ is overdrive intensity in thefirst frame period and the second frame period, andV₂₅₁₁=V_(OD2511)−V_(S2511) and V₂₅₁₂=V_(OD2512)−V_(S2512) are satisfied.

Although details of a controlling method shown in FIG. 25C are similarto those shown in FIG. 25B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₂₅₁₁ and L₂₅₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₂₅₀₁and L₂₅₀₂ in FIG. 25B. Accordingly, the original voltage and overdrivevoltage V may be changed in order to correct luminance differencedepending on a scan position. As a method for correcting originalvoltage, the method shown in FIG. 23B can be used. As a method forcorrecting overdrive voltage, the method shown in FIG. 23D can be used.Thus, color shading and motion blur in displaying a moving image can bereduced.

Next, the case of using (3) the method where a backlight divided intoareas sequentially blinks among the methods where voltage written to apixel is controlled under a condition that backlight luminance isconstant when the backlight is lit is described with reference to FIGS.26A to 26C.

FIG. 26A is a graph showing timing of writing data and timing ofsequentially blinking a backlight on the same time axis with respect toa position of a scan line when the lighting ratio R is different in thefirst frame period and the second frame period. Here, for explanation, abacklight lighting period in the first frame period is denoted byτ_(a2601), and a backlight lighting period in the second frame period isdenoted by τ_(a2602).

FIG. 26B is a graph showing original voltages V_(S2601) and V_(S2602)and overdrive voltages V_(OD2601) and V_(OD2602) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.26A. Each area of oblique line regions L₂₆₀₁ and L₂₆₀₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₆₀₁ and voltage V₂₆₀₂ is overdrive intensity in thefirst frame period and the second frame period, andV₂₆₀₁=V_(OD2601)−V_(S2601) and V₂₆₀₂=V_(OD2602)−V_(S2602) are satisfied.

In the case of using (3) the method where a backlight divided into areassequentially blinks among the methods where voltage written to a pixelis controlled under a condition that backlight luminance is constantwhen the backlight is lit before and after the lighting ratio R ischanged, driving can be realized by changing the length and timing ofthe backlight lighting period in the first frame period and the secondframe period, as shown in FIG. 26A. In addition, a relation between thevoltage written to each pixel at this time and transmittance can beunderstood with reference to FIG. 26B.

When the overdrive voltage V_(OD2601) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S2601) at the time when the next data is written by data writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is sequentially provided foreach area. Thus, integrated luminance in the top area in the first frameperiod is represented by the area of the oblique line region L₂₆₀₁,which is surrounded by the backlight lighting period and thetransmittance.

Then, when the overdrive voltage V_(OD2602) is written to the pixel bydata writing scanning in the second frame period, the transmittance ofthe display element becomes transmittance corresponding to the originalvoltage V_(S2602) at the time when the next data is written by datawriting scanning in the next frame period or at the time close thereto.In that period, a backlight lighting period is sequentially provided foreach area. Thus, integrated luminance in the top area in the secondframe period is represented by the area of the oblique line regionL₂₆₀₂, which is surrounded by the backlight lighting period and thetransmittance.

At this time, it is important that values of the voltages applied to apixel vary in the first frame period and the second frame period. Thatis, in the case where the lighting ratio R is changed under a conditionthat backlight luminance is constant when a backlight is lit, it ispreferable to write different voltage in the first frame period and thesecond frame period, not the same voltage, if luminance of the pixelwhich human eyes perceive is not desired to be changed.

Accordingly, in one of methods according to this document, the originalvoltage and the overdrive voltage are changed in accordance with thelighting ratio R in order that the area of the oblique line region L₂₆₀₁in the first frame period and the area of the oblique line region L₂₆₀₂in the second frame period are approximately the same. Specifically, itis preferable to reduce the original voltage and the overdrive voltageas the lighting ratio R increases. In addition, in one of the methodsaccording to this document, the overdrive intensity V₂₆₀₁ in the firstframe period and the overdrive intensity V₂₆₀₂ in the second frameperiod may be changed in accordance with the lighting ratio R.Specifically, it is preferable to reduce the overdrive intensity as thelighting ratio R increases. This is because increase in the lightingratio R means increase in length of the image display period τ_(a), andincrease in length of the image display period τ_(a) can be allowed tohave a longer period of time for reaching intended transmittance of aliquid crystal element. Moreover, when the length of the image displayperiod τ_(a) is increased, intended transmittance of a liquid crystalelement itself can be reduced, so that the original voltage V_(S) isreduced, and further, the overdrive intensity can be reduced.

By driving a display device in such a manner, backlight luminance can beconstant even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit for driving a backlight issimplified, so that manufacturing cost can be reduced. Further,luminance unevenness and a flicker in displaying an image can bereduced. Moreover, provision of the blanking interval τ_(b) can reducemotion blur, and image quality of a moving image can be improved.

FIG. 26C is a graph showing original voltages V_(S2611) and V_(S2612)and overdrive voltages V_(OD2611) and V_(OD2612) written to each pixel,and transmittance with respect to each voltage on the same time axis ina pixel connected to a scan line different from that shown in FIG. 26Bwhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.26A. Each area of oblique line regions L₂₆₁₁ and L₂₆₁₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₆₁₁ and voltage V₂₆₁₂ is overdrive intensity in thefirst frame period and the second frame period, andV₂₆₁₁=V_(OD2611)−V_(S2611) and V₂₆₁₂=V_(OD2612)−V_(S2612) are satisfied.

Although details of a controlling method shown in FIG. 26C are similarto those shown in FIG. 26B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₂₆₁₁ and L₂₆₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₂₆₀₁and L₂₆₀₂ in FIG. 26B. Accordingly, the original voltage and theoverdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Note that the driving methods shown in FIGS. 26B and 26C are similar inother areas. At this time, the amount of correction of the originalvoltage and the overdrive voltage in a pixel to which data is written atthe end of each area is made to be the largest in the area to which thepixel belongs, so that sharp luminance difference at a boundary of areascan be corrected.

Next, the case of a combination of (2) the method where the wholebacklight blinks and (1) the method where blanking data is directlywritten to each pixel among the methods where voltage written to a pixelis controlled under a condition that backlight luminance is constantwhen the backlight is lit is described with reference to FIGS. 27A to27C.

FIG. 27A is a graph showing timing of writing data, timing of blinking abacklight, and timing of writing blanking date on the same time axiswith respect to a position of a scan line when the lighting ratio R isdifferent in the first frame period and the second frame period. Here,for explanation, a backlight lighting period in the first frame periodis denoted by τ_(a2701), and a backlight lighting period in the secondframe period is denoted by τ_(a2702).

FIG. 27B is a graph showing original voltages V_(S2701) and V_(S2702)and overdrive voltages V_(OD2701) and V_(OD2702) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.27A. Each area of oblique line regions L₂₇₀₁ and L₂₇₀₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₇₀₁ and voltage V₂₇₀₂ is overdrive intensity in theimage display periods in the first frame period and the second frameperiod, and V₂₇₀₁=V_(OD2701)−V_(S2701) and V₂₇₀₂=V_(OD2702)−V_(S2702)are satisfied.

In the case of the combination of (2) the method where the wholebacklight blinks and (1) the method where blanking data is directlywritten to each pixel among the methods where voltage written to a pixelis controlled under a condition that backlight luminance is constantwhen the backlight is lit before and after the lighting ratio R ischanged, driving can be realized by changing the length and timing ofthe backlight lighting period in the first frame period and the secondframe period and performing blanking writing scanning in addition todata writing scanning, as shown in FIG. 27A. Here, although the case isshown in which data writing scanning and blanking writing scanning areperformed at the same timing in each frame period, a driving methodaccording to this document is not limited thereto, and various types ofwriting timing can be used. For example, data writing scanning may bechanged in accordance with the lighting ratio R. As a method where datawriting scanning is changed in accordance with the lighting ratio R, thelength of time from blanking writing scanning to data writing scanningin the same frame period may be increased as the lighting ratio R isdecreased. A relation between the voltage written to each pixel at thistime and transmittance can be understood with reference to FIG. 27B.

When the overdrive voltage V_(OD2701) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S2701) at the time when the next data is written by blanking writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is provided in all pixels allat once. Thus, integrated luminance in the first frame period isrepresented by the area of the oblique line region L₂₇₀₁, which issurrounded by the backlight lighting period and the transmittance.

Then, when the overdrive voltage V_(OD2702) is written to the pixel bydata writing scanning in the second frame period after blanking writingscanning in the second frame period, the transmittance of the displayelement becomes transmittance corresponding to the original voltageV_(S2702) at the time when the next data is written by blanking writingscanning in the next frame period or at the time close thereto. In thatperiod, a backlight lighting period is provided in all the pixels all atonce. Thus, integrated luminance in the second frame period isrepresented by the area of the oblique line region L₂₇₀₂, which issurrounded by the backlight lighting period and the transmittance.

At this time, it is important that values of the voltages applied to apixel vary in the first frame period and the second frame period. Thatis, in the case where the lighting ratio R is changed under a conditionthat backlight luminance is constant when a backlight is lit, it ispreferable to write different voltage in the first frame period and thesecond frame period, not the same voltage, if luminance of the pixelwhich human eyes perceive is not desired to be changed.

Accordingly, in one of methods according to this document, the originalvoltage and the overdrive voltage are changed in accordance with thelighting ratio R in order that the area of the oblique line region L₂₇₀₁in the first frame period and the area of the oblique line region L₂₇₀₂in the second frame period are approximately the same. Specifically, itis preferable to reduce the original voltage and the overdrive voltageas the lighting ratio R increases. In addition, in one of the methodsaccording to this document, the overdrive intensity V₂₇₀₁ in the firstframe period and the overdrive intensity V₂₇₀₂ in the second frameperiod may be changed in accordance with the lighting ratio R.Specifically, it is preferable to reduce the overdrive intensity as thelighting ratio R increases. This is because increase in the lightingratio R means increase in length of the image display period τ_(a), andincrease in length of the image display period τ_(a) can be allowed tohave a longer period of time for reaching intended transmittance of aliquid crystal element. Moreover, when the length of the image displayperiod τ_(a) is increased, intended transmittance of a liquid crystalelement itself can be reduced, so that the original voltage V_(S) isreduced, and further, the overdrive intensity can be reduced.

By driving a display device in such a manner, backlight luminance can beconstant even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit for driving a backlight issimplified, so that manufacturing cost can be reduced. Further,luminance unevenness and a flicker in displaying an image can bereduced. Moreover, provision of the blanking interval τ_(b) can reducemotion blur, and image quality of a moving image can be improved.Furthermore, since blanking writing is performed in a period other thanthe backlight lighting period, light leakage can be reduced. Thus, blackblurring in displaying an image can be reduced, so that a contrast ratioof the display device can be improved.

FIG. 27C is a graph showing original voltages V_(S2711) and V_(S2712)and overdrive voltages V_(OD2711) and V_(OD2712) written to each pixel,and transmittance with respect to each voltage on the same time axis ina pixel connected to a scan line different from that shown in FIG. 27Bwhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.27A. Each area of oblique line regions L₂₇₁₁ and L₂₇₁₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₇₁₁ and voltage V₂₇₁₂ is overdrive intensity in thefirst frame period and the second frame period, andV₂₇₁₁=V_(OD2711)−V_(S2711) and V₂₇₁₂=V_(OD2712)−V_(S2712) are satisfied.

Although details of a controlling method shown in FIG. 27C are similarto those shown in FIG. 27B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₂₇₁₁ and L₂₇₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₂₇₀₁and L₂₇₀₂ in FIG. 27B. Accordingly, the original voltage and theoverdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Next, the case of a combination of (3) the method where a backlightdivided into areas sequentially blinks and (1) the method where blankingdata is directly written to each pixel among the methods where voltagewritten to a pixel is controlled under a condition that backlightluminance is constant when the backlight is lit is described withreference to FIGS. 28A to 28C.

FIG. 28A is a graph showing timing of writing data, timing of writingblank data, and timing of sequentially blinking a backlight on the sametime axis with respect to a position of a scan line when the lightingratio R is different in the first frame period and the second frameperiod. Here, for explanation, a backlight lighting period in the firstframe period is denoted by τ_(a2801), and a backlight lighting period inthe second frame period is denoted by τ_(a2802).

FIG. 28B is a graph showing original voltages V_(S2801) and V_(S2802)and overdrive voltages V_(OD2801) and V_(OD2802) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.28A. Each area of oblique line regions L₂₈₀₁ and L₂₈₀₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₈₀₁ and voltage V₂₈₀₂ is overdrive intensity in theimage display periods in the first frame period and the second frameperiod, and V₂₈₀₁=V_(OD2801)−V_(S2801) and V₂₈₀₂=V_(OD2802)−V_(S2802)are satisfied.

In the case of the combination of (3) the method where a backlightdivided into areas sequentially blinks and (1) the method where blankingdata is directly written to each pixel among the methods where voltagewritten to a pixel is controlled under a condition that backlightluminance is constant when the backlight is lit before and after thelighting ratio R is changed, driving can be realized by changing thelength and timing of the backlight lighting period in the first frameperiod and the second frame period and performing blanking writingscanning in addition to data writing scanning, as shown in FIG. 28A.Here, although the case is shown in which data writing scanning andblanking writing scanning are performed at the same timing in each frameperiod, a driving method according to this document is not limitedthereto, and various types of writing timing can be used. For example,data writing scanning may be changed in accordance with the lightingratio R. As a method where data writing scanning is changed inaccordance with the lighting ratio R, the length of time from blankingwriting scanning to data writing scanning in the same frame period maybe increased as the lighting ratio R is decreased. A relation betweenthe voltage written to each pixel at this time and transmittance can beunderstood with reference to FIG. 28B.

When the overdrive voltage V_(OD2801) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S2801) at the time when the next data is written by blanking writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is sequentially provided foreach area. Thus, integrated luminance in the top area in the first frameperiod is represented by the area of the oblique line region L₂₈₀₁,which is surrounded by the backlight lighting period and thetransmittance.

Then, when the overdrive voltage V_(OD2802) is written to the pixel bydata writing scanning in the second frame period after blanking writingscanning in the second frame period, the transmittance of the displayelement becomes transmittance corresponding to the original voltageV_(S2802) at the time when the next data is written by data writingscanning in the next frame period or at the time close thereto. In thatperiod, a backlight lighting period is sequentially provided for eacharea. Thus, integrated luminance in the top area in the second frameperiod is represented by the area of the oblique line region L₂₈₀₂,which is surrounded by the backlight lighting period and thetransmittance.

At this time, it is important that values of the voltages applied to apixel vary in the first frame period and the second frame period. Thatis, in the case where the lighting ratio R is changed under a conditionthat backlight luminance is constant when a backlight is lit, it ispreferable to write different voltage in the first frame period and thesecond frame period, not the same voltage, if luminance of the pixelwhich human eyes perceive is not desired to be changed.

Accordingly, in one of methods according to this document, the originalvoltage and the overdrive voltage are changed in accordance with thelighting ratio R in order that the area of the oblique line region L₂₈₀₁in the first frame period and the area of the oblique line region L₂₈₀₂in the second frame period are approximately the same. Specifically, itis preferable to reduce the original voltage and the overdrive voltageas the lighting ratio R increases. In addition, in one of the methodsaccording to this document, the overdrive intensity V₂₈₀₁ in the firstframe period and the overdrive intensity V₂₈₀₂ in the second frameperiod may be changed in accordance with the lighting ratio R.Specifically, it is preferable to reduce the overdrive intensity as thelighting ratio R increases. This is because increase in the lightingratio R means increase in length of the image display period τ_(a), andincrease in length of the image display period τ_(a) can be allowed tohave a longer period of time for reaching intended transmittance of aliquid crystal element. Moreover, when the length of the image displayperiod τ_(a) is increased, intended transmittance of a liquid crystalelement itself can be reduced, so that the original voltage V_(S) isreduced, and further, the overdrive intensity can be reduced.

By driving a display device in such a manner, backlight luminance can beconstant even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit for driving a backlight issimplified, so that manufacturing cost can be reduced. Further,luminance unevenness and a flicker in displaying an image can bereduced. Moreover, provision of the blanking interval τ_(b) can reducemotion blur, and image quality of a moving image can be improved.Furthermore, since blanking writing is performed in a period other thanthe backlight lighting period, light leakage in a non-lighting period ofthe backlight can be reduced. Thus, black blurring in displaying animage can be reduced, so that a contrast ratio of the display device canbe improved.

FIG. 28C is a graph showing original voltages V_(S2811) and V_(S2812)and overdrive voltages V_(OD2811) and V_(OD2812) written to each pixel,and transmittance with respect to each voltage on the same time axis ina pixel connected to a scan line different from that shown in FIG. 28Bwhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.28A. Each area of oblique line regions L₂₈₁₁ and L₂₈₁₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₂₈₁₁ and voltage V₂₈₁₂ is overdrive intensity in thefirst frame period and the second frame period, andV₂₈₁₁=V_(OD2811)−V_(S2811) and V₂₈₁₂=V_(OD2812)−V_(S2812) are satisfied.

Although details of a controlling method shown in FIG. 28C are similarto those shown in FIG. 28B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₂₈₁₁ and L₂₈₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₂₈₀₁and L₂₈₀₂ in FIG. 28B. Accordingly, the original voltage and theoverdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Note that the driving methods shown in FIGS. 28B and 28C are similar inother areas. At this time, the amount of correction of the originalvoltage and the overdrive voltage in a pixel to which data is written atthe end of each area is made to be the largest in the area to which thepixel belongs, so that sharp luminance difference at a boundary of areascan be corrected.

Next, in a method of changing backlight luminance, the case where themethod for providing the blanking interval τ_(b) (the method ofcontrolling the lighting ratio R) is different is individually describedin detail. Note that in the method of changing backlight luminance, bycontrolling transmittance of a display element, backlight luminance canhave extremely various values in order to prevent change in luminancewhich human eyes perceive in frame periods before and after the lightingratio R is changed. Here, the case where voltage written to each pixelis not changed when the lighting ratio R is changed is described. Thisis because this can provide a beneficial effect in driving a displaydevice.

First, the case of using (2) the method where the whole backlight blinksamong the methods of changing backlight luminance is described withreference to FIGS. 29A to 29C.

FIG. 29A is a graph showing timing of writing data and timing ofblinking a backlight on the same time axis with respect to a position ofa scan line when the lighting ratio R is different in the first frameperiod and the second frame period. Here, for explanation, a backlightlighting period in the first frame period is denoted by τ_(a2901), and abacklight lighting period in the second frame period is denoted byτ_(a2902).

FIG. 29B is a graph showing original voltage V_(S2901) and overdrivevoltage V_(OD2901) written to each pixel, and transmittance with respectto each voltage on the same time axis when the lighting ratio R isdifferent in the first frame period and the second frame period. Here,the backlight lighting periods in the first frame period and the secondframe period are similar to those in FIG. 29A. Each area of oblique lineregions L₂₉₀₁ and L₂₉₀₂ represents the level of luminance which humaneyes perceive (integrated luminance). Voltage V₂₉₀₁ is overdriveintensity in the first frame period, and V₂₉₀₁=V_(OD2901)−V_(S2901) issatisfied.

In the case of using (2) the method where the whole backlight blinksamong the methods of changing backlight luminance before and after thelighting ratio R is changed, driving can be realized by changingbacklight luminance and the length and timing of the backlight lightingperiod, as shown in FIG. 29A. A relation between the voltage written toeach pixel at this time and transmittance can be understood withreference to FIG. 29B.

When the overdrive voltage V_(OD2901) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S2901) at the time when the next data is written by data writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is provided in all pixels allat once. Thus, integrated luminance in the first frame period isrepresented by the area of the oblique line region L₂₉₀₁, which issurrounded by the backlight lighting period and the transmittance.

In the second frame period, the transmittance in the graph of FIG. 29Bis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of the oblique line region L₂₉₀₂ in the second frame period.

At this time, it is important that luminance in the backlight lightingperiods varies in the first frame period and the second frame period.That is, when the lighting ratio R is changed, display can be performedwithout change in luminance of a pixel which human eyes perceive bychanging backlight luminance even in the case where luminance of thepixel is not desired to be changed.

Accordingly, in one of methods according to this document, backlightluminance in the backlight lighting period is determined by differencebetween the area of the oblique line region L₂₉₀₁ in the first frameperiod and the area of the oblique line region L₂₉₀₂ in the second frameperiod. Specifically, when the lighting ratio R is changed and thebacklight lighting period in the second frame period is 1/X (X is apositive number) of the backlight lighting period in the first frameperiod, it is preferable that backlight luminance be X times as high asthat in the first frame period. Then, in one of the methods according tothis document, it is preferable that the original voltage V_(S2901) inthe first frame period be approximately the same in the first frameperiod and the second frame period.

By driving a display device in such a manner, when the lighting ratio Ris changed, the original voltage V_(S2901) can be approximately the samein the first frame period and the second frame period even in the casewhere luminance of the pixel which human eyes perceive is not desired tobe changed. Thus, a structure of a circuit which processes image dataand is included in the display device is simplified, so thatmanufacturing cost and power consumption of the display device can bereduced. Further, in the case where the same luminance is desired to bedisplayed when the lighting ratio R is changed, voltage written to eachpixel does not have to be changed from that in the previous frame; thus,power consumption in writing data can be reduced.

Note that overdrive voltage and overdrive intensity do not have to beapproximately the same in the first frame period and the second frameperiod. This is because overdrive voltage and overdrive intensity areobtained from original voltages and transmittance in one frame and theprevious frame; thus, when original voltage and transmittance in eachprevious frame are different in the first frame period and the secondframe period, various values are obtained as a matter of course.

FIG. 29C is a graph showing original voltage V_(S2911) and overdrivevoltage V_(OD2911) written to each pixel, and transmittance with respectto each voltage on the same time axis in a pixel connected to a scanline different from that shown in FIG. 29B when the lighting ratio R isdifferent in the first frame period and the second frame period. Here,the backlight lighting periods in the first frame period and the secondframe period are similar to those in FIG. 29A. Each area of oblique lineregions L₂₉₁₁ and L₂₉₁₂ represents the level of luminance which humaneyes perceive (integrated luminance). Voltage V₂₉₁₁ is overdriveintensity in the first frame period, and V₂₉₁₁=V_(OD2911)−V_(S2911) issatisfied.

Although details of a controlling method shown in FIG. 29C are similarto those shown in FIG. 29B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₂₉₁₁ and L₂₉₁₂, which representsintegrated, luminance, is different from each area of the oblique lineregions L₂₉₀₁ and L₂₉₀₂ in FIG. 29B. Accordingly, the original voltageand the overdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Next, the case of using (3) the method where a backlight divided intoareas sequentially blinks among the methods of changing backlightluminance is described with reference to FIGS. 30A to 30C.

FIG. 30A is a graph showing timing of writing data and timing ofsequentially blinking a backlight on the same time axis with respect toa position of scan lines when the lighting ratio R is different in thefirst frame period and the second frame period. Here, for explanation, abacklight lighting period in the first frame period is denoted byτ_(a3001), and a backlight lighting period in the second frame period isdenoted by τ_(a3002).

FIG. 30B is a graph showing original voltage V_(S3001) and overdrivevoltage V_(OD3001) written to each pixel, and transmittance with respectto each voltage on the same time axis when the lighting ratio R isdifferent in the first frame period and the second frame period. Here,the backlight lighting periods in the first frame period and the secondframe period are similar to those in FIG. 30A. Each area of oblique lineregions L₃₀₀₁ and L₃₀₀₂ represents the level of luminance which humaneyes perceive (integrated luminance). Voltage V₃₀₀₁ is overdriveintensity in the first frame period, and V₃₀₀₁=V_(OD3001)−V_(S3001) issatisfied.

In the case of using (3) the method where a backlight divided into areassequentially blinks among the methods of changing backlight luminancebefore and after the lighting ratio R is changed, driving can berealized by changing backlight luminance and the length and timing ofthe backlight lighting period, as shown in FIG. 30A. A relation betweenthe voltage written to each pixel at this time and transmittance can beunderstood with reference to FIG. 30B.

When the overdrive voltage V_(OD3001) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S3001) at the time when the next data is written by data writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is sequentially provided foreach area. Thus, integrated luminance in the top area in the first frameperiod is represented by the area of the oblique line region L₃₀₀₁,which is surrounded by the backlight lighting period and thetransmittance.

In the second frame period, the transmittance in the graph of FIG. 30Bis already desired transmittance before data is written. At this time,the transmittance dose not change in the backlight lighting period.Luminance which human eyes perceive in the second frame period dependson the area of the oblique line region L₃₀₀₂ in the second frame period.

At this time, it is important that luminance in the backlight lightingperiods varies in the first frame period and the second frame period.That is, when the lighting ratio R is changed, display can be performedwithout change in luminance of a pixel which human eyes perceive bychanging backlight luminance even in the case where luminance of thepixel is not desired to be changed.

Accordingly, in one of methods according to this document, backlightluminance in the backlight lighting period is determined by differencebetween the area of the oblique line region L₃₀₀₁ in the first frameperiod and the area of the oblique line region L₃₀₀₂ in the second frameperiod. Specifically, when the lighting ratio R is changed and thebacklight lighting period in the second frame period is 1/X (X is apositive number) of the backlight lighting period in the first frameperiod, it is preferable that backlight luminance be X times as high asthat in the first frame period. Then, in one of the methods according tothis document, it is preferable that the original voltage V_(S3001) inthe first frame period be approximately the same in the first frameperiod and the second frame period.

By driving a display device in such a manner, when the lighting ratio Ris changed, the original voltage V_(S3001) can be approximately the samein the first frame period and the second frame period even in the casewhere luminance of the pixel which human eyes perceive is not desired tobe changed. Thus, a structure of a circuit which processes image dataand is included in the display device is simplified, so thatmanufacturing cost and power consumption of the display device can bereduced. Further, in the case where the same luminance is desired to bedisplayed when the lighting ratio R is changed, voltage written to eachpixel does not have to be changed from that in the previous frame; thus,power consumption in writing data can be reduced.

Note that overdrive voltage and overdrive intensity do not have to beapproximately the same in the first frame period and the second frameperiod. This is because overdrive voltage and overdrive intensity areobtained from original voltages and transmittance in one frame and theprevious frame; thus, when original voltage and transmittance in eachprevious frame are different in the first frame period and the secondframe period, various values are obtained as a matter of course.

FIG. 30C is a graph showing original voltage V_(S3011) and overdrivevoltage V_(OD3011) written to each pixel, and transmittance with respectto each voltage on the same time axis in a pixel connected to a scanline different from that shown in FIG. 30B when the lighting ratio R isdifferent in the first frame period and the second frame period. Here,the backlight lighting periods in the first frame period and the secondframe period are similar to those in FIG. 30A. Each area of oblique lineregions V₃₀₁₁ and L₃₀₁₂ represents the level of luminance which humaneyes perceive (integrated luminance). Voltage V₃₀₁₁ is overdriveintensity in the first frame period, and V₃₀₁₁=V_(OD3011)−V_(S3011) issatisfied.

Although details of a controlling method shown in FIG. 30C are similarto those shown in FIG. 30B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₃₀₁₁ and L₃₀₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₃₀₀₁and L₃₀₀₂ in FIG. 30B. Accordingly, the original voltage and theoverdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Note that the driving methods shown in FIGS. 30B and 30C are similar inother areas. At this time, the amount of correction of the originalvoltage and the overdrive voltage in a pixel to which data is written atthe end of each area is made to be the largest in the area to which thepixel belongs, so that sharp luminance difference at a boundary of areascan be corrected.

Next, the case of a combination of (2) the method where the wholebacklight blinks and (1) the method where blanking data is directlywritten to each pixel among the methods of changing backlight luminanceis described with reference to FIGS. 31A to 31C.

FIG. 31A is a graph showing timing of writing data, timing of writingblank data, and timing of blinking a backlight on the same time axiswith respect to a position of a scan line when the lighting ratio R isdifferent in the first frame period and the second frame period. Here,for explanation, a backlight lighting period in the first frame periodis denoted by τ_(a3101), and a backlight lighting period in the secondframe period is denoted by τ_(a3102).

FIG. 31B is a graph showing original voltage V_(S3101) and overdrivevoltages V_(OD3101) and V_(OD3102) written to each pixel, andtransmittance with respect to each voltage on the same time axis whenthe lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.31A. Each area of oblique line regions L₃₁₀₁ and L₃₁₀₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₃₁₀₁ and voltage V₃₁₀₂ is overdrive intensity in imagedisplay periods in the first frame period and the second frame period,and V₃₁₀₁=V_(OD3101)−V_(S3101) and V₃₁₀₂=V_(OD3102)−V_(S3101) aresatisfied.

In the case of the combination of (2) the method where the wholebacklight blinks and (1) the method where blanking data is directlywritten to each pixel among the methods of changing backlight luminancebefore and after the lighting ratio R is changed, driving can berealized by changing backlight luminance and the length and timing ofthe backlight lighting period, and performing blanking writing scanningin addition to data writing scanning, as shown in FIG. 31A. Here,although the case is shown in which data writing scanning and blankingwriting scanning are performed at the same timing in each frame period,a driving method according to this document is not limited thereto, andvarious types of writing timing can be used. For example, data writingscanning may be changed in accordance with the lighting ratio R. As amethod where data writing scanning is changed in accordance with thelighting ratio R, the length of time from blanking writing scanning todata writing scanning in the same frame period may be increased as thelighting ratio R is decreased. A relation between the voltage written toeach pixel at this time and transmittance can be understood withreference to FIG. 31B.

When the overdrive voltage V_(OD3101) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S3101) at the time when the next data is written by blanking writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is provided in all pixels allat once. Thus, integrated luminance in the first frame period isrepresented by the area of the oblique line region L₃₁₀₁, which issurrounded by the backlight lighting period and the transmittance.

Then, when the overdrive voltage V_(OD3102) is written to the pixel bydata writing scanning in the second frame period after blanking writingscanning in the second frame period, the transmittance of the displayelement becomes transmittance corresponding to the original voltageV_(S3101) at the time when the next data is written by blanking writingscanning in the next frame period or at the time close thereto. In thatperiod, a backlight lighting period is provided in all the pixel all atonce. Thus, integrated luminance in the second frame period isrepresented by the area of the oblique line region L₃₁₀₂, which issurrounded by the backlight lighting period and the transmittance.

At this time, it is important that luminance in the backlight lightingperiods varies in the first frame period and the second frame period.That is, when the lighting ratio R is changed, display can be performedwithout change in luminance of a pixel which human eyes perceive bychanging backlight luminance even in the case where luminance of thepixel is not desired to be changed.

Accordingly, in one of methods according to this document, backlightluminance in the backlight lighting period is determined by differencebetween the area of the oblique line region L₃₁₀₁ in the first frameperiod and the area of the oblique line region L₃₁₀₂ in the second frameperiod. Specifically, when the lighting ratio R is changed and thebacklight lighting period in the second frame period is 1/X (X is apositive number) of the backlight lighting period in the first frameperiod, it is preferable that backlight luminance be X times as high asthat in the first frame period. Then, in one of the methods according tothis document, it is preferable that the original voltage V_(S3101) inthe first frame period be approximately the same in the first frameperiod and the second frame period.

By driving a display device in such a manner, the original voltageV_(S3101) can be the same in the first frame period and the second frameperiod even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit which processes image data,which is included in the display device, is simplified, so thatmanufacturing cost and power consumption of the display device can bereduced. Further, since blanking writing is performed in a period otherthan the backlight lighting period, light leakage in a non-lightingperiod of the backlight can be reduced. Thus, black blurring indisplaying an image can be reduced, so that a contrast ratio of thedisplay device can be improved.

Note that overdrive voltage and overdrive intensity do not have to beapproximately the same in the first frame period and the second frameperiod. This is because overdrive voltage and overdrive intensity areobtained from original voltages and transmittance in one frame and theprevious frame; thus, when original voltage and transmittance in eachprevious frame are different in the first frame period and the secondframe period, various values are obtained as a matter of course.

FIG. 31C is a graph showing original voltage V_(S3111) and overdrivevoltages V_(OD3111) and V_(OD3111) written to each pixel, andtransmittance with respect to each voltage on the same time axis in apixel connected to a scan line different from that shown in FIG. 31Bwhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.31A. Each area of oblique line regions L₃₁₁₁ and L₃₁₁₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₃₁₁₁ and voltage V₃₁₁₂ is overdrive intensity in thefirst frame period and the second frame period, andV₃₁₁₁=V_(OD3111)−V_(S3111) and V₃₁₁₂ V_(OD3112)−V_(S3111) are satisfied.

Although details of a controlling method shown in FIG. 31C are similarto those shown in FIG. 31B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₃₁₁₁ and L₃₁₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₃₁₀₁and L₃₁₀₂ in FIG. 31B. Accordingly, the original voltage and theoverdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Next, the case of a combination of (3) the method where a backlightdivided into areas sequentially blinks and (1) the method where blankingdata is directly written to each pixel among the methods of changingbacklight luminance is described with reference to FIGS. 32A to 32C.

FIG. 32A is a graph showing timing of writing data, timing of writingblank data, and timing of sequentially blinking a backlight on the sametime axis with respect to a position of a scan line when the lightingratio R is different in the first frame period and the second frameperiod. Here, for explanation, a backlight lighting period in the firstframe period is denoted by τ_(a3201), and a backlight lighting period inthe second frame period is denoted by τ_(a3202).

FIG. 32B is a graph showing original voltage V_(S3201) and overdrivevoltages V_(OD3201) and V_(OD3202) written to each pixel, andtransmittance with respect to each voltage on the same time axis whenthe lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.32A. Each area of oblique line regions L₃₂₀₁ and L₃₂₀₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₃₂₀₁ and voltage V₃₂₀₂ is overdrive intensity in imagedisplay periods in the first frame period and the second frame period,and V₃₂₀₁=V_(OD3201)−V_(S3201) and V₃₂₀₂=V_(OD3202)−V_(S3201) aresatisfied.

In the case of the combination of (3) the method where a backlightdivided into areas sequentially blinks and (1) the method where blankingdata is directly written to each pixel among the methods of changingbacklight luminance before and after the lighting ratio R is changed,driving can be realized by changing backlight luminance and the lengthand timing of the backlight lighting period, and performing blankingwriting scanning in addition to data writing scanning, as shown in FIG.32A. Here, although the case is shown in which data writing scanning andblanking writing scanning are performed at the same timing in each frameperiod, a driving method according to this document is not limitedthereto, and various types of writing timing can be used. For example,data writing scanning may be changed in accordance with the lightingratio R. As a method where data writing scanning is changed inaccordance with the lighting ratio R, the length of time from blankingwriting scanning to data writing scanning in the same frame period maybe increased as the lighting ratio R is decreased. A relation betweenthe voltage written to each pixel at this time and transmittance can beunderstood with reference to FIG. 32B. In addition, although timing ofdata writing scanning and blanking writing scanning does not overlapwith the backlight lighting period in the graph of FIG. 32A, a methodaccording to this document is not limited thereto, and the timingthereof may overlap with the backlight lighting period. For example,writing scanning and blanking scanning at all scan positions may overlapwith the backlight lighting periods. In this case, the backlight isalready lit when data is written or at the time close thereto, and at oraround the time when blanking data is written, a blanking intervalstarts even when the backlight is lit. Accordingly, time from whenwriting is performed to when the backlight lighting period starts is thesame at all the scan positions, so that luminance difference of pixelsdepending on a scan position disappears, and luminance unevenness indisplaying an image can be reduced. Further, since a period when thebacklight is not lit is in the blanking interval, light leakage in theblanking interval can be reduced. Thus, black blurring in displaying animage can be reduced, so that a contrast ratio of the display device canbe improved. Moreover, the length of the blanking interval τ_(b) can becontrolled by changing timing of blanking writing, instead of changing astate of sequential scanning of the backlight so that the length of thebacklight lighting period is changed. At this time, since timing ofblanking writing can be changed in each one gate selection period, thelength of the blanking interval τ_(b) can be finely adjusted, and thedegree of reduction in motion blur can be finely changed. Accordingly,the lighting ratio R depending on the control parameters P and Q can befurther optimally controlled.

When the overdrive voltage V_(OD3201) is written to a pixel by datawriting scanning in the first frame period, transmittance of a displayelement becomes transmittance corresponding to the original voltageV_(S3201) at the time when the next data is written by blanking writingscanning in the second frame period or at the time close thereto. Inthat period, a backlight lighting period is sequentially provided foreach area. Thus, integrated luminance in the top area in the first frameperiod is represented by the area of the oblique line region L₃₂₀₁,which is surrounded by the backlight lighting period and thetransmittance.

Then, when the overdrive voltage V_(OD3202) is written to the pixel bydata writing scanning in the second frame period after blanking writingscanning in the second frame period, the transmittance of the displayelement becomes transmittance corresponding to the original voltageV_(S3201) at the time when the next data is written by blanking writingscanning in the next frame period or at the time close thereto. In thatperiod, a backlight lighting period is sequentially provided for eacharea. Thus, integrated luminance in the top area in the second frameperiod is represented by the area of the oblique line region L₃₂₀₂,which is surrounded by the backlight lighting period and thetransmittance.

At this time, it is important that luminance in the backlight lightingperiods varies in the first frame period and the second frame period.That is, when the lighting ratio R is changed, display can be performedwithout change in luminance of a pixel which human eyes perceive bychanging backlight luminance even in the case where luminance of thepixel is not desired to be changed.

Accordingly, in one of methods according to this document, backlightluminance in a backlight lighting period is determined by differencebetween the area of the oblique line region L₃₂₀₁ in the first frameperiod and the area of the oblique line region L₃₂₀₂ in the second frameperiod. Specifically, when the lighting ratio R is changed and thebacklight lighting period in the second frame period is 1/X (X is apositive number) of the backlight lighting period in the first frameperiod, it is preferable that backlight luminance be X times as high asthat in the first frame period. Then, in one of the methods according tothis document, it is preferable that the original voltage V_(S3201) inthe first frame period be approximately the same in the first frameperiod and the second frame period.

By driving a display device in such a manner, the original voltageV_(S3201) can be the same in the first frame period and the second frameperiod even in the case where luminance of the pixel which human eyesperceive is not desired to be changed when the lighting ratio R ischanged. Thus, a structure of a circuit which processes image data,which is included in the display device, is simplified, so thatmanufacturing cost and power consumption of the display device can bereduced. Further, since blanking writing is performed in a period otherthan the backlight lighting period, light leakage in a non-lightingperiod of the backlight can be reduced. Thus, black blurring indisplaying an image can be reduced, so that a contrast ratio of thedisplay device can be improved.

Note that overdrive voltage and overdrive intensity do not have to beapproximately the same in the first frame period and the second frameperiod. This is because overdrive voltage and overdrive intensity areobtained from original voltages and transmittance in one frame and theprevious frame; thus, when original voltage and transmittance in eachprevious frame are different in the first frame period and the secondframe period, various values are obtained as a matter of course.

FIG. 32C is a graph showing original voltage V_(S3211) and overdrivevoltages V_(OD3211) and V_(OD3212) written to each pixel, andtransmittance with respect to each voltage on the same time axis in apixel connected to a scan line different from that shown in FIG. 32Bwhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, the backlight lighting periods in the firstframe period and the second frame period are similar to those in FIG.32A. Each area of oblique line regions L₃₂₁₁ and L₃₂₁₂ represents thelevel of luminance which human eyes perceive (integrated luminance).Each of voltage V₃₂₁₁ and voltage V₃₂₁₂ is overdrive intensity in thefirst frame period and the second frame period, andV₃₂₁₁=V_(OD3211)−V_(S3211) and V₃₂₁₂=V_(OD3212)−V_(S3211) are satisfied.

Although details of a controlling method shown in FIG. 32C are similarto those shown in FIG. 32B, the length of time from when data is writtento when a backlight lighting period starts is different. Thus, each areaof the oblique line regions L₃₂₁₁ and L₃₂₁₂, which represents integratedluminance, is different from each area of the oblique line regions L₃₂₀₁and L₃₂₀₂ in FIG. 32B. Accordingly, the original voltage and theoverdrive voltage V may be changed in order to correct luminancedifference depending on a scan position. As a method for correctingoriginal voltage, the method shown in FIG. 23B can be used. As a methodfor correcting overdrive voltage, the method shown in FIG. 23D can beused. Thus, color shading and motion blur in displaying a moving imagecan be reduced.

Note that the driving methods shown in FIGS. 32B and 32C are similar inother areas. At this time, the amount of correction of the originalvoltage and the overdrive voltage in a pixel to which data is written atthe end of each area is made to be the largest in the area to which thepixel belongs, so that sharp luminance difference at a boundary of areascan be corrected.

At least one of the methods of driving a display device according tothis document can be used when a pixel provided in the display deviceincludes a plurality of subpixels. At this time, reduction in displayquality, such as motion blur, can be further reduced by driving with thelighting ratio R different in each subpixel.

When a pixel includes a plurality of subpixels, a function of the pixelcan be extended, and properties of a display device can be improved. Forexample, the number of gray scales which the pixel can display can beincreased by changing luminance in each subpixel and combining suchluminance (i.e., area gray scale). In addition, when a display elementis a liquid crystal element, there are problems such as reduction incontrast of display, color shift, and luminance inversion depending onan angle at which a display portion of the display device is seen (i.e.,a narrow viewing angle). When the pixel includes a plurality ofsubpixels and voltages slightly different from each other are applied toeach subpixel, a viewing angle of the display device can be increased.Accordingly, various beneficial effects can be obtained by a structurewhere each pixel provided in the display device includes a plurality ofsubpixels, and properties of the display device can be further improvedby using the method described in this embodiment mode.

An example of a pixel including a plurality of subpixels is describedwith reference to FIG. 33A. A pixel 3350 shown in FIG. 33A includes afirst subpixel 3351 and a second subpixel 3352. Here, the first subpixel3351 and the second subpixel 3352 are also referred to as a subpixel Iand a subpixel II.

A plurality of wirings are connected to the first subpixel 3351 and thesecond subpixel 3352, and various connection methods can be used. As astructure example of wirings connected to a plurality of subpixels, astructure shown in FIG. 33A can be used, for example. In the structureshown in FIG. 33A, a data line DATA which is a signal line fortransmitting a data signal is connected to the plurality of subpixels incommon. Further, scan lines GATEI_(n) and GATEII_(n) which are signallines for selecting the subpixel I and the subpixel II are separatelyconnected to respective subpixels. Here, n is a positive integerrepresenting the number of scan lines.

For a pixel structure, various structures other than the structure shownin FIG. 33A can be used. For example, the data lines DATA may beseparately connected to a plurality of subpixels, and a scan line GATEmay be connected to the plurality of subpixels in common. Alternatively,both the data lines DATA and the scan lines GATE may be separatelyconnected to a plurality of subpixels. Here, description of structuresother than the structure shown in FIG. 33A is omitted.

Note that the structures shown in FIGS. 9G and 9H can be used for theinside of the first subpixel 3351 and the second subpixel 3352.

As a method where a data signal is written to each subpixel, sequentialscanning is usually performed. That is, GATEI₁, GATEII₁, GATEI₂, andGATEII₂ are sequentially selected, GATEI_(X) and GATEII_(X) areselected, and scanning finishes. Here, X represents the number of pixelsin a perpendicular direction. This sequential scanning may be performedwhen writing scanning and blanking scanning are performed.

When writing scanning and blanking scanning are performed by a scanningmethod shown in FIG. 33B, driving with the lighting ratio R different ineach subpixel can be realized.

FIG. 33B is a timing chart with a horizontal axis representing time anda vertical axis representing voltage with respect to each signal line.The data line DATA represents voltage written to a pixel. The scan linesGATEI_(n) and GATEII_(n) represent a non-selected state when at lowlevel and a selected state when at high level.

In the scanning method shown in FIG. 33B, one gate selection period isdivided into two periods, and the first half of one gate selectionperiod represents a period in which a data signal is written to a pixeland the latter half thereof represents a period in which blanking datais written. In the first half of one gate selection period, a datasignal is written to each pixel by sequentially scanning scan lines,whereas in the latter half of one gate selection period, the scan linesmay be scanned with timing depending on the lighting ratio R of eachsubpixel without sequential scanning of the scan lines.

Specifically, after a data signal is written to GATEI₁, a data signal iswritten to GATEII₁ in the first half of the next gate selection period.Next, GATEI₂ and GATEII₂ are sequentially selected and scanned. Then, atthe time when an image display period of GATEI₁ ends, blanking data iswritten to GATEI₁ in the latter half of the gate selection period. Then,at the time when an image display period of GATEII₁ ends, blanking datais written to GATEII₁ in the latter half of the gate selection period.In such a manner, writing scanning is sequentially performed andtemporally-discrete blanking scanning is performed on each subpixel, sothat driving with the lighting ratio R different in each subpixel can berealized. Further, an image display period τ_(a3301) of the scan lineGATEI_(n) at this time is a period from writing scanning to blankingscanning, and a blanking interval τ_(b3301) is a period from blankingscanning to writing scanning in the next frame. Similarly, an imagedisplay period τ_(a3311) of the scan line GATEII_(n) is a period fromwriting scanning to blanking scanning, and a blanking interval τ_(b3311)is a period from blanking scanning to writing scanning in the nextframe.

Here, a data signal is written to a pixel in the first half of one gateselection period, and blanking data is written to the pixel in thelatter half thereof; on the contrary, blanking data may be written to apixel in the first half of one gate selection period and a data signalmay be written to the pixel in the latter half thereof.

Voltage V_(blank) of blanking data may vary in a period when blankingdata is written to the subpixel I and a period when blanking data iswritten to the subpixel II. Accordingly, luminance of a pixel in theblanking interval may freely vary in each subpixel.

In particular, a method where the lighting ratio R can vary in eachsubpixel is beneficial to a display device in which a viewing angle isincreased by displaying a bright image in one of subpixels and a darkimage in the other of the subpixels. This is because an effect ofreducing motion blur can be obtained in a bright pixel in which motionblur is likely to be seen and a gray scale on the lower gray scale levelcan be sufficiently displayed in a dark pixel in which a gray scale onthe lower gray scale level is likely to be damaged, by reducing thelighting ratio R in a subpixel for displaying a bright image andincreasing the lighting ratio R in a subpixel for displaying a darkimage.

As an example where the lighting ratio R freely varies in each subpixel,the length of image display periods τ_(a3401) and τ_(a3402) of thesubpixel I can be different from the length of image display periodsτ_(a3411) and τ_(a3412) of the subpixel II, as shown in FIGS. 34A and34B. Accordingly, an effect of reducing motion blur can be obtained in abright pixel in which motion blur is likely to be seen and a gray scaleon the lower gray scale level can be sufficiently displayed in a darkpixel in which a gray scale on the lower gray scale level is likely tobe damaged.

FIG. 34A is a graph showing timing of writing data and timing of writingblanking date in the first frame period and the second frame period onthe same time axis with respect to a position of a scan line. The imagedisplay periods of the subpixel I in the first frame period and thesecond frame period are denoted by τ_(a3401) and τ_(a3402). Blankingintervals of the subpixel I in the first frame period and the secondframe period are denoted by τ_(b3401) and τ_(b3402). The image displayperiods of the subpixel II in the first frame period and the secondframe period are denoted by τ_(a3411) and τ_(a3412). Blanking intervalsof the subpixel II in the first frame period and the second frame periodare denoted by τ_(b3411) and τ_(b3412).

FIG. 34BI is a graph showing original voltage V_(S3401) and overdrivevoltages V_(OD3401) and V_(OD3402) written to each pixel, andtransmittance with respect to each voltage in the first frame period andthe second frame period on the same time axis. Here, the image displayperiods and the blanking intervals in the first frame period and thesecond frame period are similar to those in FIG. 34A. Each of voltageV₃₄₀₁ and voltage V voltage V₃₄₀₂ is overdrive intensity in the imagedisplay periods in the first frame period and the second frame period,and V₃₄₀₁=V_(OD3401)−V_(S3401) and V₃₄₀₂=V_(OD3402)−V_(S3401) aresatisfied.

FIG. 34BII is a graph showing original voltage V_(S3411) and overdrivevoltages V_(OD3411) and V_(OD3412) written to each pixel, andtransmittance with respect to each voltage in the first frame period andthe second frame period on the same time axis. Here, the image displayperiods and the blanking intervals in the first frame period and thesecond frame period are similar to those in FIG. 34A. Each of voltageV₃₄₁₁ and voltage V voltage V₃₄₁₂ is overdrive intensity in the imagedisplay periods in the first frame period and the second frame period,and V₃₄₁₁=V_(OD3411)−V_(S3411) and V₃₄₁₂=V_(OD3412)−V_(S3411) aresatisfied.

As shown in FIG. 34B, when the lighting ratio R varies in the subpixel Iand the subpixel II, it is preferable to reduce the overdrive intensityin each frame as the lighting ratio R increases. This is becauseincrease in the lighting ratio R means increase in length of the imagedisplay period τ_(a), and increase in length of the image display periodτ_(a) can be allowed to have a longer period of time for reachingintended transmittance of a liquid crystal element. Moreover, when thelength of the image display period τ_(a) is increased, intendedtransmittance of a liquid crystal element itself can be reduced, so thatthe original voltage V_(S) is reduced, and further, the overdriveintensity can be reduced.

Note that transmittance at or around the time when each frame endschanges depending on the length of the blanking interval. Specifically,the transmittance at or around the time when each frame ends increasesas the blanking interval is reduced. Thus, it is preferable to furtherreduce overdrive intensity of one frame as the blanking interval of theprevious frame is shorter.

In addition, difference in the lighting ratio R of the subpixels I andII is preferably determined in accordance with the control parameter P.Specifically, it is preferable to increase difference in the lightingratio R of the subpixels I and II as the control parameter P increases.This is because an effect of reducing motion blur can be obtained in abright pixel in which motion blur is likely to be seen, whereas a grayscale on the lower gray scale level can be sufficiently displayed in adark pixel in which a gray scale on the lower gray scale level is likelyto be damaged.

Other examples of a method where the lighting ratio R can vary betweensubpixels include a method where the lighting ratio R is changed in oneof subpixels and not changed in the other of the subpixels in accordancewith the magnitude of the control parameters P and Q (see FIGS. 35A and35B), and a method where the lighting ratio R is changed in one ofsubpixels and is also changed in the other of the subpixels inaccordance with the magnitude of the control parameters P and Q (seeFIGS. 36A and 36B). Thus, an optimal driving method in accordance with astate of an image can be set. Specifically, since a bright subpixel canincrease the whole luminance and has a property that motion blur islikely to be seen, it is preferable to reduce the lighting ratio R asthe control parameter P increases. Since a dark subpixel cannotsufficiently display a gray scale on the lower gray scale level and hasa property that motion blur is not likely to be seen, motion blur ishardly seen even when the control parameter P increases. Thus, when thecontrol parameter P increases, the lighting ratio R can be increased. Byincreasing the lighting ratio R, a gray scale on the lower gray scalelevel can be sufficiently displayed in a dark pixel in which a grayscale on the lower gray scale level is likely to be damaged.Accordingly, it is very beneficial to optimally control the lightingratio R with respect to the control parameter P depending on propertiesof each subpixel.

Note that optimal driving can also be realized by changing backlightluminance at this time. For example, when the level of a data signalwritten to a pixel is the same, luminance which human eyes perceivebecomes lower as the image display period τ_(a) becomes shorter and theblanking interval τ_(b) becomes longer. Accordingly, in accordance withthe length of the image display period τ_(a) and the length of theblanking interval τ_(b) (i.e., the lighting ratio R), the backlightluminance is reduced when the lighting ratio R is high, whereas thebacklight luminance is increased when the lighting ratio R is low; thus,luminance which human eyes perceive can be constant. Further, thelighting ratio R preferably depends on the control parameters P and Qdescribed in Embodiment Mode 3. This is because the lighting ratio R canbe controlled as appropriate by perceivability of motion blur in animage to be displayed.

FIG. 35A is a graph showing timing of writing data and timing of writingblanking data in the first frame period and the second frame period onthe same time axis with respect to a position of a scan line. Imagedisplay periods of the subpixel I in the first frame period and thesecond frame period are denoted by τ_(a3501) and τ_(a3502). Blankingintervals of the subpixel I in the first frame period and the secondframe period are denoted by τ_(b3501) and τ_(b3509). Image displayperiods of the subpixel II in the first frame period and the secondframe period are denoted by τ_(a3511) and τ_(a3512). Blanking intervalsof the subpixel II in the first frame period and the second frame periodare denoted by τ_(b3511) and τ_(b3512).

FIG. 35BI is a graph showing original voltages V_(S3501) and V_(S3502)and overdrive voltages V_(OD3501) and V_(OD3502) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, image display periods and blanking intervalsin the first frame period and the second frame period are similar tothose in FIG. 35A. Each area of oblique line regions L₃₅₀₁ and L₃₅₀₂represents the level of luminance which human eyes perceive (integratedluminance). Each of voltage V₃₅₀₁ and voltage V₃₅₀₂ is overdriveintensity in the image display periods in the first frame period and thesecond frame period, and V₃₅₀₁=V_(OD3501)−V_(S3501) andV₃₅₀₂=V_(OD3502)−V_(S3502) are satisfied.

FIG. 35BII is a graph showing original voltage V_(S3511) and overdrivevoltages V_(OD3511) and V_(OD3512) written to each pixel, andtransmittance with respect to each voltage in the first frame period andthe second frame period on the same time axis. Here, image displayperiods and blanking intervals in the first frame period and the secondframe period are similar to those in FIG. 35A. Each of voltage V₃₅₁₁ andvoltage V₃₅₁₂ is overdrive intensity in the image display periods in thefirst frame period and the second frame period, andV₃₅₁₁=V_(OD3511)−V_(S3511) and V₃₅₁₂=V_(OD3512)−V_(S3511) are satisfied.

In FIG. 35BI, the area of the oblique line region L₃₅₀₁ and the area ofthe oblique line region L₃₅₀₂ are made approximately the same bycontrolling the original voltage and the overdrive voltage asappropriate, so that luminance which human eyes perceive can beapproximately the same even when the lighting ratio R is different.

FIG. 36A is a graph showing timing of writing data and timing of writingblanking data in the first frame period and the second frame period onthe same time axis with respect to a position of a scan line. Imagedisplay periods of the subpixel I in the first frame period and thesecond frame period are denoted by τ_(a3601) and τ_(a3602). Blankingintervals of the subpixel I in the first frame period and the secondframe period are denoted by τ_(b3601) and τ_(b3602). Image displayperiods of the subpixel II in the first frame period and the secondframe period are denoted by τ_(a3611) and τ_(a3612). Blanking intervalsof the subpixel II in the first frame period and the second frame periodare denoted by τ_(b3611) and τ_(b3612).

FIG. 36BI is a graph showing original voltages V_(S3601) and V_(S3602)and overdrive voltages V_(OD3601) and V_(OD3602) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, image display periods and blanking intervalsin the first frame period and the second frame period are similar tothose in FIG. 36A. Each area of oblique line regions L₃₆₀₁ and L₃₆₀₂represents the level of luminance which human eyes perceive (integratedluminance). Each of voltage V₃₆₀₁ and voltage V₃₆₀₂ is overdriveintensity in the image display periods in the first frame period and thesecond frame period, and V₃₆₀₁=V_(OD3601)−V_(S3601) andV₃₆₀₂=V_(OD3602)−V_(S3602) are satisfied.

FIG. 36BII is a graph showing original voltages V_(S3611) and V_(S3612)and overdrive voltages V_(OD3611) and V_(OD3612) written to each pixel,and transmittance with respect to each voltage on the same time axiswhen the lighting ratio R is different in the first frame period and thesecond frame period. Here, backlight lighting periods and blankingintervals in the first frame period and the second frame period aresimilar to those in FIG. 36A. Each area of oblique line regions L₃₆₁₁and L₃₆₁₂ represents the level of luminance which human eyes perceive(integrated luminance). Each of voltage V₃₆₁₁ and voltage V₃₆₁₂ isoverdrive intensity in image display periods in the first frame periodand the second frame period, and V₃₆₁₁=V_(OD3611)−V_(S3611) andV₃₆₁₂=V_(OD3612)−V_(S3612) are satisfied.

In FIG. 36BI, the area of the oblique line region L₃₆₀₁ and the area ofthe oblique line region L₃₆₀₂ are made approximately the same bycontrolling the original voltage and the overdrive voltage asappropriate, so that luminance which human eyes perceive can beapproximately the same even when the lighting ratio R is different.

In FIG. 36BII also, the area of the oblique line region L₃₆₁₁ and thearea of the oblique line region L₃₆₁₂ are made approximately the same bycontrolling the original voltage and the overdrive voltage asappropriate, so that luminance which human eyes perceive can beapproximately the same even when the lighting ratio R is different.

Note that the methods shown in FIGS. 35A, 35B, 36A, and 36B, it iseffective to combine the method where the lighting ratio R freely variesbetween subpixels with the control parameter P described in anotherembodiment mode. For example, by increasing difference between thelighting ratios R of the subpixels as the control parameter P increases,an effect of reducing motion blur can be obtained in a bright pixel inwhich motion blur is likely to be seen and a gray scale on the lowergray scale level can be sufficiently displayed in a dark pixel in whicha gray scale on the lower gray scale level is likely to be damaged.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 5

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a liquid crystal displaydevice is described.

A pixel structure in the case where each liquid crystal mode and atransistor are combined is described with reference to cross-sectionalviews of a pixel.

Note that as the transistor, a thin film transistor (a TFT) or the likeincluding a non-single crystalline semiconductor layer typified byamorphous silicon, polycrystalline silicon, micro crystalline (alsoreferred to as semi-amorphous) silicon, or the like can be used.

As a structure of the transistor, a top-gate structure, a bottom-gatestructure, or the like can be used. Note that a channel-etchedtransistor, a channel-protective transistor, or the like can be used asa bottom-gate transistor.

FIG. 37 is an example of across-sectional view of a pixel in the casewhere a TN mode and a transistor are combined. By applying the pixelstructure shown in FIG. 37 to a liquid crystal display device, a liquidcrystal display device can be formed at low cost.

Features of the pixel structure shown in FIG. 37 are described. Liquidcrystal molecules 10118 shown in FIG. 37 are long and narrow moleculeseach having a major axis and a minor axis. In FIG. 37, a direction ofeach of the liquid crystal molecules 10118 is expressed by the lengththereof. That is, the direction of the major axis of the liquid crystalmolecule 10118, which is expressed as long, is parallel to the page, andas the liquid crystal molecule 10118 is expressed to be shorter, thedirection of the major axis becomes closer to a normal direction of thepage. That is, among the liquid crystal molecules 10118 shown in FIG.37, the direction of the major axis of the liquid crystal molecule 10118which is close to the first substrate 10101 and the direction of themajor axis of the liquid crystal molecule 10118 which is close to thesecond substrate 10116 are different from each other by 90 degrees, andthe directions of the major axes of the liquid crystal molecules 10118located therebetween are arranged so as to link the above two directionssmoothly. That is, the liquid crystal molecules 10118 shown in FIG. 37are aligned to be twisted by 90 degrees between the first substrate10101 and the second substrate 10116.

Here, the case is described in which a bottom-gate transistor using anamorphous semiconductor is used as the transistor. In the case where atransistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.37, the two substrates correspond to the first substrate 10101 and thesecond substrate 10116. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10114, a color filter10115, a fourth conductive layer 10113, a spacer 10117, and a secondalignment film 10112 are formed on the second substrate.

The light-shielding film 10114 is not necessarily formed on the secondsubstrate 10116. When the light-shielding film 10114 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.In addition, since a structure is simple, yield can be improved. On theother hand, when the light-shielding film 10114 is formed, a displaydevice with little light leakage at the time of black display can beobtained.

The color filter 10115 is not necessarily formed on the second substrate10116. When the color filter 10115 is not formed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple, yield can be improved. Note that even when thecolor filter 10115 is not formed, a display device which can performcolor display can be obtained by field sequential driving. On the otherhand, needless to say, when the color filter 10115 is formed, a displaydevice which can perform color display can be obtained.

Spherical spacers may be dispersed on the second substrate 10116 insteadof forming the spacer 10117. When the spherical spacers are dispersed,the number of steps is reduced, so that manufacturing cost can bereduced. In addition, since a structure is simple, yield can beimproved. On the other hand, when the spacer 10117 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

Subsequently, a process to be performed to the first substrate 10101 isdescribed.

First, a first insulating film 10102 is formed over the first substrate10101 by sputtering, a printing method, a coating method, or the like.The first insulating film 10102 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects a semiconductor layer. Note that the first insulating film10102 is not necessarily formed.

Next, a first conductive layer 10103 is formed over the first insulatingfilm 10102 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10104 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10104 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects the semiconductor layer.

Next, a first semiconductor layer 10105 and a second semiconductor layer10106 are formed. Note that the first semiconductor layer 10105 and thesecond semiconductor layer 10106 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10107 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which processes a shape of the second conductivelayer 10107, dry etching is preferable. Note that as the secondconductive layer 10107, a light-transmitting material may be used or areflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 10106 isetched by using the second conductive layer 10107 as a mask.Alternatively, the second semiconductor layer 10106 is etched by using amask for processing the shape of the second conductive layer 10107.Then, the first conductive layer 10103 at a position where the secondsemiconductor layer 10106 is removed serves as the channel region of thetransistor. Thus, the number of masks can be reduced, so thatmanufacturing cost can be reduced.

Next, a third insulating film 10108 is formed and a contact hole isselectively formed in the third insulating film 10108. Note that acontact hole may be formed also in the second insulating film 10104 atthe same time as forming the contact hole in the third insulating film10108. Note also that a surface of the third insulating film 10108 ispreferably as even as possible. This is because alignment of the liquidcrystal molecules are affected by unevenness of a surface with which theliquid crystal is in contact.

Next, a third conductive layer 10109 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 10110 is formed. Note that after the firstalignment film 10110 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10101 which is manufactured as described above andthe second substrate 10116 on which the light-shielding film 10114, thecolor filter 10115, the fourth conductive layer 10113, the spacer 10117,and the second alignment film 10112 are formed are attached to eachother by a sealant with a gap of several μm therebetween. Then, liquidcrystals 10111 which include the liquid crystal molecules 10118 areinjected into a space between the two substrates. Note that in the TNmode, the fourth conductive layer 10113 is formed over the entiresurface of the second substrate 10116.

FIG. 38A is an example of a cross-sectional view of a pixel in the casewhere an MVA (Multi-domain Vertical Alignment) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 38A to a liquidcrystal display device, a liquid crystal display device having a wideviewing angle, high response speed, and high contrast can be obtained.

Features of the pixel structure of an MVA-mode liquid crystal panelshown in FIG. 38A are described. Liquid crystal molecules 10218 shown inFIG. 38A are long and narrow molecules each having a major axis and aminor axis. In FIG. 38A, a direction of each of the liquid crystalmolecules 10218 is expressed by the length thereof. That is, thedirection of the major axis of the liquid crystal molecule 10218, whichis expressed as long, is parallel to the page, and as the liquid crystalmolecule 10218 is expressed to be shorter, the direction of the majoraxis becomes closer to a normal direction of the page. That is, theliquid crystal molecules 10218 shown in FIG. 38A are aligned such thatthe direction of the major axis is normal to the alignment film. Thus,the liquid crystal molecules 10218 at a position where an alignmentcontrol projection 10219 is formed are aligned radially with thealignment control projection 10219 as a center. With this state, aliquid crystal display device having a wide viewing angle can beobtained.

Here, the case is described in which a bottom-gate transistor using anamorphous semiconductor is used as the transistor. In the case where atransistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.38A, the two substrates correspond to the first substrate 10201 and thesecond substrate 10216. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10214, a color filter10215, a fourth conductive layer 10213, a spacer 10217, a secondalignment film 10212, and an alignment control projection 10219 areformed on the second substrate.

The light-shielding film 10214 is not necessarily formed on the secondsubstrate 10216. When the light-shielding film 10214 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.In addition, since a structure is simple, yield can be improved. On theother hand, when the light-shielding film 10214 is formed, a displaydevice with little light leakage at the time of black display can beobtained.

The color filter 10215 is not necessarily formed on the second substrate10216. When the color filter 10215 is not formed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple, yield can be improved. Note that even when thecolor filter 10215 is not formed, a display device which can performcolor display can be obtained by field sequential driving. On the otherhand, needless to say, when the color filter 10215 is formed, a displaydevice which can perform color display can be obtained.

Spherical spacers may be dispersed on the second substrate 10216 insteadof forming the spacer 10217. When the spherical spacers are dispersed,the number of steps is reduced, so that manufacturing cost can bereduced. In addition, since a structure is simple, yield can beimproved. On the other hand, when the spacer 10217 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

Subsequently, a process to be performed to the first substrate 10201 isdescribed.

First, a first insulating film 10202 is formed over the first substrate10201 by sputtering, a printing method, a coating method, or the like.The first insulating film 10202 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects a semiconductor layer. Note that the first insulating film10202 is not necessarily formed.

Next, a first conductive layer 10203 is formed over the first insulatingfilm 10202 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10204 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10204 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects the semiconductor layer.

Next, a first semiconductor layer 10205 and a second semiconductor layer10206 are formed. Note that the first semiconductor layer 10205 and thesecond semiconductor layer 10206 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10207 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which processes a shape of the second conductivelayer 10207, dry etching is preferable. Note that as the secondconductive layer 10207, a light-transmitting material may be used or areflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 10206 isetched by using the second conductive layer 10207 as a mask.Alternatively, the second semiconductor layer 10206 is etched by using amask for processing the shape of the second conductive layer 10207.Then, the first conductive layer 10203 at a position where the secondsemiconductor layer 10206 is removed serves as the channel region of thetransistor. Thus, the number of masks can be reduced, so thatmanufacturing cost can be reduced.

Next, a third insulating film 10208 is formed and a contact hole isselectively formed in the third insulating film 10208. Note that acontact hole may be formed also in the second insulating film 10204 atthe same time as forming the contact hole in the third insulating film10208.

Next a third conductive layer 10209 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 10210 is formed. Note that after the firstalignment film 10210 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10201 which is manufactured as described above andthe second substrate 10216 on which the light-shielding film 10214, thecolor filter 10215, the fourth conductive layer 10213, the spacer 10217,and the second alignment film 10212 are manufactured are attached toeach other by a sealant with a gap of several μm therebetween. Then,liquid crystals 10211 which include the liquid crystal molecules 10218are injected into a space between the two substrates. Note that in theMVA mode, the fourth conductive layer 10213 is formed over the entiresurface of the second substrate 10216. In addition, the alignmentcontrol projection 10219 is formed so as to be in contact with thefourth conductive layer 10213. The alignment control projection 10219preferably has a shape with a smooth curved surface. Thus, sincealignment of the adjacent liquid crystal molecules 10218 is extremelysimilar, an alignment defect can be reduced. Further, a defect of thealignment film caused by breaking of the alignment film can be reduced.

FIG. 38B is an example of a cross-sectional view of a pixel in the casewhere a PVA (Patterned Vertical Alignment) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 38B to a liquidcrystal display device, a liquid crystal display device having a wideviewing angle, high response speed, and high contrast can be obtained.

Features of the pixel structure shown in FIG. 38B are described. Liquidcrystal molecules 10248 shown in FIG. 38B are long and narrow moleculeseach having a major axis and a minor axis. In FIG. 38B, direction ofeach of the liquid crystal molecules 10248 is expressed by the lengththereof. That is, the direction of the major axis of the liquid crystalmolecule 10248, which is expressed as long, is parallel to the page, andas the liquid crystal molecule 10248 is expressed to be shorter, thedirection of the major axis becomes closer to a normal direction of thepage. That is, the liquid crystal molecules 10248 shown in FIG. 38B arealigned such that the direction of the major axis is normal to thealignment film. Thus, the liquid crystal molecules 10248 at a positionwhere an electrode cutout portion 10249 is formed are aligned radiallywith a boundary of the electrode cutout portion 10249 and the fourthconductive layer 10243 as a center. With this state, a liquid crystaldisplay device having a wide viewing angle can be obtained.

Here, the case is described in which a bottom-gate transistor using anamorphous semiconductor is used as the transistor. In the case where atransistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.38B, the two substrates correspond to the first substrate 10231 and thesecond substrate 10246. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10244, a color filter10245, a fourth conductive layer 10243, a spacer 10247, and a secondalignment film 10242 are formed on the second substrate.

The light-shielding film 10244 is not necessarily formed on the secondsubstrate 10246. When the light-shielding film 10244 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.In addition, since a structure is simple, yield can be improved. On theother hand, when the light-shielding film 10244 is formed, a displaydevice with little light leakage at the time of black display can beobtained.

The color filter 10245 is not necessarily formed on the second substrate10246. When the color filter 10245 is not formed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple, yield can be improved. Note that even when thecolor filter 10245 is not formed, a display device which can performcolor display can be obtained by field sequential driving. On the otherhand, needless to say, when the color filter 10245 is formed, a displaydevice which can perform color display can be obtained.

Spherical spacers may be dispersed on the second substrate 10246 insteadof forming the spacer 10247. When the spherical spacers are dispersed,the number of steps is reduced, so that manufacturing cost can bereduced. In addition, since a structure is simple, yield can beimproved. On the other hand, when the spacer 10247 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

Subsequently, a process to be performed to the first substrate 10231 isdescribed.

First, a first insulating film 10232 is formed over the first substrate10231 by sputtering, a printing method, a coating method, or the like.The first insulating film 10232 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects a semiconductor layer. Note that the first insulating film10232 is not necessarily formed.

Next, a first conductive layer 10233 is formed over the first insulatingfilm 10232 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10234 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10234 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects the semiconductor layer.

Next, a first semiconductor layer 10235 and a second semiconductor layer10236 are formed. Note that the first semiconductor layer 10235 and thesecond semiconductor layer 10236 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10237 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which processes a shape of the second conductivelayer 10237, dry etching is preferable. Note that as the secondconductive layer 10237, a light-transmitting material may be used or areflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 10236 isetched by using the second conductive layer 10237 as a mask.Alternatively, the second semiconductor layer 10236 is etched by using amask for processing the shape of the second conductive layer 10237.Then, the first conductive layer 10233 at a position where the secondsemiconductor layer 10236 is removed serves as the channel region of thetransistor. Thus, the number of masks can be reduced, so thatmanufacturing cost can be reduced.

Next, a third insulating film 10238 is formed and a contact hole isselectively formed in the third insulating film 10238. Note that acontact hole may be formed also in the second insulating film 10234 atthe same time as forming the contact hole in the third insulating film10238. Note also that a surface of the third insulating film 10238 ispreferably as even as possible. This is because alignment of the liquidcrystal molecules are affected by unevenness of a surface with which theliquid crystal is in contact.

Next, a third conductive layer 10239 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a first alignment film 10240 is formed. Note that after the firstalignment film 10240 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10231 which is manufactured as described above andthe second substrate 10246 on which the light-shielding film 10244, thecolor filter 10245, the fourth conductive layer 10243, the spacer 10247,and the second alignment film 10242 are manufactured are attached toeach other by a sealant with a gap of several μm therebetween. Then,liquid crystals 10241 which include the liquid crystal molecules 10248are injected into a space between the two substrates. Note that in thePVA mode, the fourth conductive layer 10243 is patterned and is providedwith the electrode cutout portion 10249. Although a shape of theelectrode cutout portion 10249 is not particularly limited, theelectrode cutout portion 10249 preferably has a shape in which aplurality of rectangles having different directions are combined. Thus,since a plurality of regions having different alignment can be formed, aliquid crystal display device having a wide viewing angle can beobtained. Note that the fourth conductive layer 10243 at the boundarybetween the electrode cutout portion 10249 and the fourth conductivelayer 10243 preferably has a shape with a smooth curved surface. Thus,since alignment of the adjacent liquid crystal molecules 10248 isextremely similar, an alignment defect is reduced. Further, a defect ofthe alignment film caused by breaking of the second alignment film 10242by the electrode cutout portion 10249 can be prevented.

FIG. 39A is an example of a cross-sectional view of a pixel in the casewhere an IPS (In-Plane-Switching) mode and a transistor are combined. Byapplying the pixel structure shown in FIG. 39A to a liquid crystaldisplay device, a liquid crystal display device theoretically having awide viewing angle and response speed which has low dependency on a grayscale can be obtained.

Features of the pixel structure shown in FIG. 39A are described. Liquidcrystal molecules 10318 shown in FIG. 39A are long and narrow moleculeseach having a major axis and a minor axis. In FIG. 39A, a direction ofeach of the liquid crystal molecules 10318 is expressed by the lengththereof. That is, the direction of the major axis of the liquid crystalmolecule 10318, which is expressed as long, is parallel to the page, andas the liquid crystal molecule 10318 is expressed to be shorter, thedirection of the major axis becomes closer to a normal direction of thepage. Each of the liquid crystal molecules 10318 shown in FIG. 39A isaligned so that the direction of the major axis thereof is alwayshorizontal to the substrate. Although FIG. 39A shows alignment with noelectric field, when an electric field is applied to each of the liquidcrystal molecules 10318, each of the liquid crystal molecules 10318rotates in a horizontal plane as the direction of the major axis thereofis always horizontal to the substrate. With this state, a liquid crystaldisplay device having a wide viewing angle can be obtained.

Here, the case is described in which a bottom-gate transistor using anamorphous semiconductor is used as the transistor. In the case where atransistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.39A, the two substrates correspond to the first substrate 10301 and thesecond substrate 10316. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10314, a color filter10315, a fourth conductive layer 10313, a spacer 10317, and a secondalignment film 10312 are formed on the second substrate.

The light-shielding film 10314 is not necessarily formed on the secondsubstrate 10316. When the light-shielding film 10314 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.In addition, since a structure is simple, yield can be improved. On theother hand, when the light-shielding film 10314 is formed, a displaydevice with little light leakage at the time of black display can beobtained.

The color filter 10315 is not necessarily formed on the second substrate10316. When the color filter 10315 is not formed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple, yield can be improved. Note that even when thecolor filter 10315 is not formed, a display device which can performcolor display can be obtained by field sequential driving. On the otherhand, needless to say, when the color filter 10315 is formed, a displaydevice which can perform color display can be obtained.

Spherical spacers may be dispersed on the second substrate 10316 insteadof forming the spacer 10317. When the spherical spacers are dispersed,the number of steps is reduced, so that manufacturing cost can bereduced. In addition, since a structure is simple, yield can beimproved. On the other hand, when the spacer 10317 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

Subsequently, a process to be performed to the first substrate 10301 isdescribed.

First, a first insulating film 10302 is formed over the first substrate10301 by sputtering, a printing method, a coating method, or the like.The first insulating film 10302 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects a semiconductor layer. Note that the first insulating film10302 is not necessarily formed.

Next, a first conductive layer 10303 is formed over the first insulatingfilm 10302 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10304 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10304 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects the semiconductor layer.

Next, a first semiconductor layer 10305 and a second semiconductor layer10306 are formed. Note that the first semiconductor layer 10305 and thesecond semiconductor layer 10306 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10307 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which processes a shape of the second conductivelayer 10307, dry etching is preferable. Note that as the secondconductive layer 10307, a light-transmitting material may be used or areflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 10306 isetched by using the second conductive layer 10307 as a mask.Alternatively, the second semiconductor layer 10306 is etched by using amask for processing the shape of the second conductive layer 10307.Then, the first conductive layer 10303 at a position where the secondsemiconductor layer 10306 is removed serves as the channel region of thetransistor. Thus, the number of masks can be reduced, so thatmanufacturing cost can be reduced.

Next, a third insulating film 10308 is formed and a contact hole isselectively formed in the third insulating film 10308. Note that acontact hole may be formed also in the second insulating film 10304 atthe same time as forming the contact hole in the third insulating film10308.

Next, a third conductive layer 10309 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Here, thethird conductive layer 10309 has a shape in which two comb-shapedelectrodes engage with each other. One of the comb-shaped electrodes iselectrically connected to one of a source electrode and a drainelectrode of the transistor, and the other of the comb-shaped electrodesis electrically connected to a common electrode. Thus, a lateralelectric field can be effectively applied to the liquid crystalmolecules 10318.

Next, a first alignment film 10310 is formed. Note that after the firstalignment film 10310 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10301 which is manufactured as described above andthe second substrate 10316 on which the light-shielding film 10314, thecolor filter 10315, the spacer 10317, and the second alignment film10312 are formed are attached to each other by a sealant with a gap ofseveral μm therebetween. Then, liquid crystals 10311 which include theliquid crystal molecules 10318 are injected into a space between the twosubstrates.

FIG. 39B is an example of a cross-sectional view of a pixel in the casewhere an FFS (Fringe Field Switching) mode and a transistor arecombined. By applying the pixel structure shown in FIG. 39B to a liquidcrystal display device, a liquid crystal display device theoreticallyhaving a wide viewing angle and response speed which has low dependencyon a gray scale can be obtained.

Features of the pixel structure shown in FIG. 39B are described. Liquidcrystal molecules 10348 shown in FIG. 39B are long and narrow moleculeseach having a major axis and a minor axis. In FIG. 39B, direction ofeach of the liquid crystal molecules 10348 is expressed by the lengththereof. That is, the direction of the major axis of the liquid crystalmolecule 10348, which is expressed as long, is parallel to the page, andas the liquid crystal molecule 10348 is expressed to be shorter, thedirection of the major axis becomes closer to a normal direction of thepage. Each of the liquid crystal molecules 10348 shown in FIG. 39B isaligned so that the direction of the major axis thereof is alwayshorizontal to the substrate. Although FIG. 39B shows alignment with noelectric field, when an electric field is applied to each of the liquidcrystal molecules 10348, each of the liquid crystal molecules 10348rotates in a horizontal plane as the direction of the major axis thereofis always horizontal to the substrate. With this state, a liquid crystaldisplay device having a wide viewing angle can be obtained.

Here, the case is described in which a bottom-gate transistor using anamorphous semiconductor is used as the transistor. In the case where atransistor using an amorphous semiconductor is used, a liquid crystaldisplay device can be formed at low cost by using a large substrate.

A liquid crystal display device includes a basic portion displayingimages, which is called a liquid crystal panel. The liquid crystal panelis manufactured as follows: two processed substrates are attached toeach other with a gap of several μm therebetween, and a liquid crystalmaterial is injected into a space between the two substrates. In FIG.39B, the two substrates correspond to the first substrate 10331 and thesecond substrate 10346. A transistor and a pixel electrode are formedover the first substrate. A light-shielding film 10344, a color filter10345, a spacer 10347, and a second alignment film 10342 are formed onthe second substrate.

The light-shielding film 10344 is not necessarily formed on the secondsubstrate 10346. When the light-shielding film 10344 is not formed, thenumber of steps is reduced, so that manufacturing cost can be reduced.In addition, since a structure is simple, yield can be improved. On theother hand, when the light-shielding film 10344 is formed, a displaydevice with little light leakage at the time of black display can beobtained.

The color filter 10345 is not necessarily formed on the second substrate10346. When the color filter 10345 is not formed, the number of steps isreduced, so that manufacturing cost can be reduced. In addition, since astructure is simple, yield can be improved. Note that even when thecolor filter 10345 is not formed, a display device which can performcolor display can be obtained by field sequential driving. On the otherhand, needless to say, when the color filter 10345 is formed, a displaydevice which can perform color display can be obtained.

Spherical spacers may be dispersed on the second substrate 10346 insteadof forming the spacer 10347. When the spherical spacers are dispersed,the number of steps is reduced, so that manufacturing cost can bereduced. In addition, since a structure is simple, yield can beimproved. On the other hand, when the spacer 10347 is formed, a distancebetween the two substrates can be uniform because a position of thespacer is not varied, so that a display device with little displayunevenness can be obtained.

Subsequently, a process to be performed to the first substrate 10331 isdescribed.

First, a first insulating film 10332 is formed over the first substrate10331 by sputtering, a printing method, a coating method, or the like.The first insulating film 10332 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects a semiconductor layer. Note that the first insulating film10332 is not necessarily formed.

Next, a first conductive layer 10333 is formed over the first insulatingfilm 10332 by photolithography, a laser direct writing method, an inkjetmethod, or the like.

Next, a second insulating film 10334 is formed over the entire surfaceby sputtering, a printing method, a coating method, or the like. Thesecond insulating film 10334 has a function of preventing change incharacteristics of the transistor due to an impurity from the substratewhich affects the semiconductor layer.

Next, a first semiconductor layer 10335 and a second semiconductor layer10336 are formed. Note that the first semiconductor layer 10335 and thesecond semiconductor layer 10336 are formed sequentially and shapesthereof are processed at the same time.

Next, a second conductive layer 10337 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Note that asa method for etching which processes a shape of the second conductivelayer 10337, dry etching is preferable. Note that as the secondconductive layer 10337, a light-transmitting material may be used or areflective material may be used.

Next, a channel region of the transistor is formed. Here, an example ofa step thereof is described. The second semiconductor layer 10336 isetched by using the second conductive layer 10337 as a mask.Alternatively, the second semiconductor layer 10336 is etched by using amask for processing the shape of the second conductive layer 10337.Then, the first conductive layer 10333 at a position where the secondsemiconductor layer 10336 is removed serves as the channel region of thetransistor. Thus, the number of masks can be reduced, so thatmanufacturing cost can be reduced.

Next, a third insulating film 10338 is formed and a contact hole isselectively formed in the third insulating film 10338.

Next, a fourth conductive layer 10343 is formed by photolithography, alaser direct writing method, an inkjet method, or the like.

Next, a fourth insulating film 10349 is formed and a contact hole isselectively formed in the fourth insulating film 10349. Note that asurface of the fourth insulating film 10349 is preferably as even aspossible. This is because alignment of the liquid crystal molecules areaffected by unevenness of a surface with which the liquid crystal is incontact.

Next, a third conductive layer 10339 is formed by photolithography, alaser direct writing method, an inkjet method, or the like. Here, thethird conductive layer 10339 is comb-shaped.

Next, a first alignment film 10340 is formed. Note that after the firstalignment film 10340 is formed, rubbing may be performed so as tocontrol the alignment of the liquid crystal molecules. Rubbing is a stepof forming stripes on an alignment film by rubbing the alignment filmwith a cloth. By performing rubbing, the alignment film can havealignment properties.

The first substrate 10331 which is manufactured as described above andthe second substrate 10346 on which the light-shielding film 10344, thecolor filter 10345, the spacer 10347, and the second alignment film10342 are formed are attached to each other by a sealant with a gap ofseveral μm therebetween. Then, liquid crystals 10341 which include theliquid crystal molecules 10348 are injected into a space between the twosubstrates. Therefore, a liquid crystal panel can be manufactured.

Here, materials which can be used for conductive layers or insulatingfilms are described.

As the first insulating film 10102 in FIG. 37, the first insulating film10202 in FIG. 38A, the first insulating film 10232 in FIG. 38B, thefirst insulating film 10302 in FIG. 39A, or the first insulating film10332 in FIG. 39B, an insulating film such as a silicon oxide film, asilicon nitride film, or a silicon oxynitride film can be used.Alternatively, an insulating film having a stacked-layer structure inwhich two or more of a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and the like are combined can be used.

As the first conductive layer 10103 in FIG. 37, the first conductivelayer 10203 in FIG. 38A, the first conductive layer 10233 in FIG. 38B,the first conductive layer 10303 in FIG. 39A, or the first conductivelayer 10333 in FIG. 39B, Mo, Ti, Al, Nd, Cr, or the like can be used.Alternatively, a stacked-layer structure in which two or more of Mo, Ti,Al, Nd, Cr, and the like are combined can be used.

As the second insulating film 10104 in FIG. 37, the second insulatingfilm 10204 in FIG. 38A, the second insulating film 10234 in FIG. 38B,the second insulating film 10304 in FIG. 39A, or the second insulatingfilm 10334 in FIG. 39B, a thermal oxide film, a silicon oxide film, asilicon nitride film, a silicon oxynitride film, or the like can beused. Alternatively, a stacked-layer structure in which two or more of athermal oxide film, a silicon oxide film, a silicon nitride film, asilicon oxynitride film, and the like are combined can be used. Notethat a silicon oxide film is preferable in a portion which is in contactwith a semiconductor layer. This is because a trap level at an interfacewith the semiconductor layer decreases when a silicon oxide film isused. Note also that a silicon nitride film is preferable in a portionwhich is in contact with Mo. This is because a silicon nitride film doesnot oxidize Mo.

As the first semiconductor layer 10105 in FIG. 37, the firstsemiconductor layer 10205 in FIG. 38A, the first semiconductor layer10235 in FIG. 38B, the first semiconductor layer 10305 in FIG. 39A, orthe first semiconductor layer 10335 in FIG. 39B, silicon, silicongermanium, or the like can be used.

As the second semiconductor layer 10106 in FIG. 37, the secondsemiconductor layer 10206 in FIG. 38A, the second semiconductor layer10236 in FIG. 38B, the second semiconductor layer 10306 in FIG. 39A, orthe second semiconductor layer 10336 in FIG. 39B, silicon or the likeincluding phosphorus can be used, for example.

As a light-transmitting material of the second conductive layer 10107and the third conductive layer 10109 in FIG. 37; the second conductivelayer 10207 and the third conductive layer 10209 in FIG. 38A; the secondconductive layer 10237 and a third conductive layer 10239 in FIG. 38B;the second conductive layer 10307 and a third conductive layer 10309 inFIG. 39A; or the second conductive layer 10337, the third conductivelayer 10339, and the fourth conductive layer 10343 in FIG. 39B, anindium tin oxide film formed by mixing tin oxide into indium oxide, anindium tin silicon oxide film formed by mixing silicon oxide into indiumtin oxide, an indium zinc oxide film formed by mixing zinc oxide intoindium oxide, a zinc oxide film, a tin oxide film, or the like can beused. Note that indium zinc oxide is a light-transmitting conductivematerial formed by sputtering using a target in which zinc oxide ismixed into indium tin oxide at 2 to 20 wt %.

As a reflective material of the second conductive layer 10107 and thethird conductive layer 10109 in FIG. 37; the second conductive layer10207 and the third conductive layer 10209 in FIG. 38A; the secondconductive layer 10237 and the third conductive layer 10239 in FIG. 38B;the second conductive layer 10307 and the third conductive layer 10309in FIG. 39A; or the second conductive layer 10337, the third conductivelayer 10339, and the fourth conductive layer 10343 in FIG. 39B, Ti, Mo,Ta, Cr, W, Al, or the like can be used. Alternatively, a two-layerstructure in which Al and Ti, Mo, Ta, Cr, or W are stacked, or athree-layer structure in which Al is interposed between metals such asTi, Mo, Ta, Cr, and W may be used.

As the third insulating film 10108 in FIG. 37, the third insulating film10208 in FIG. 38A, the third insulating film 10238 in FIG. 38B, thethird insulating film 10308 in FIG. 39A, or the third insulating film10338 and the fourth insulating film 10349 in FIG. 39B, an inorganicmaterial (e.g., silicon oxide, silicon nitride, or silicon oxynitride),an organic compound material having a low dielectric constant (e.g., aphotosensitive or nonphotosensitive organic resin material), or the likecan be used. Alternatively, a material including siloxane can be used.Note that siloxane is a material in which a basic structure is formed bya bond of silicon (Si) and oxygen (O). As a substituent, an organicgroup including at least hydrogen (e.g., an alkyl group or an arylgroup) is used. Alternatively, a fluoro group may be used as thesubstituent. Further alternatively, the organic group including at leasthydrogen and the fluoro group may be used as the substituent.

As the first alignment film 10110 in FIG. 37, the first alignment film10210 in FIG. 38A, the first alignment film 10240 in FIG. 38B, the firstalignment film 10310 in FIG. 39A, or the first alignment film 10340 inFIG. 39B, a film of a high molecular compound such as polyimide can beused.

Next, the pixel structure in the case where each liquid crystal mode andthe transistor are combined is described with reference to a top planview (a layout diagram) of the pixel.

Note that as a liquid crystal mode, a TN (twisted nematic) mode, an IPS(in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA(multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASM (axially symmetric aligned micro-cell) mode, anOCB (optical compensated birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (antiferroelectric liquid crystal) mode,or the like can be used.

As the transistor, a thin film transistor (a TFT) including a non-singlecrystalline semiconductor film typified by amorphous silicon,polycrystalline silicon, microcrystalline (also referred to assemi-amorphous) silicon, or the like can be used.

As a structure of the transistor, a top-gate structure, a bottom-gatestructure, or the like can be used. A channel-etched transistor, achannel-protective transistor, or the like can be used as a bottom-gatetransistor.

FIG. 40 is an example of a top plan view of a pixel in the case where aTN mode and a transistor are combined. By applying the pixel structureshown in FIG. 40 to a liquid crystal display device, a liquid crystaldisplay device can be formed at low cost.

The pixel shown in FIG. 40 includes a scan line 10401, an image signalline 10402, a capacitor line 10403, a transistor 10404, a pixelelectrode 10405, and a pixel capacitor 10406.

The scan line 10401 has a function of transmitting a signal (a scansignal) to the pixel. The image signal line 10402 has a function fortransmitting a signal (an image signal) to the pixel. Note that sincethe scan line 10401 and the image signal line 10402 are arranged inmatrix, they are formed of conductive layers in different layers. Notealso that a semiconductor layer may be provided at an intersection ofthe scan line 10401 and the image signal line 10402. Thus, intersectioncapacitance between the scan line 10401 and the image signal line 10402can be reduced.

The capacitor line 10403 is provided in parallel to the pixel electrode10405. A portion where the capacitor line 10403 and the pixel electrode10405 overlap with each other corresponds to the pixel capacitor 10406.Note that part of the capacitor line 10403 is extended along the imagesignal line 10402 so as to surround the image signal line 10402. Thus,crosstalk can be reduced. Crosstalk is a phenomenon in which a potentialof an electrode, which should hold the potential, is changed inaccordance with change in potential of the image signal line 10402. Notealso that intersection capacitance can be reduced by providing asemiconductor layer between the capacitor line 10403 and the imagesignal line 10402. Note also that the capacitor line 10403 is formed ofa material which is similar to that of the scan line 10401.

The transistor 10404 has a function as a switch which turns on the imagesignal line 10402 and the pixel electrode 10405. Note that one of asource region and a drain region of the transistor 10404 is provided soas to be surrounded by the other of the source region and the drainregion of the transistor 10404. Thus, the channel width of thetransistor 10404 increases, so that switching capability can beimproved. Note also that a gate electrode of the transistor 10404 isprovided so as to surround the semiconductor layer.

The pixel electrode 10405 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10404. The pixelelectrode 10405 is an electrode for applying signal voltage which istransmitted by the image signal line 10402 to a liquid crystal element.Note that the pixel electrode 10405 is rectangular. Thus, an apertureratio can be improved. Note also that as the pixel electrode 10405, alight-transmitting material may be used or a reflective material may beused. Alternatively, the pixel electrode 10405 may be formed bycombining a light-transmitting material and a reflective material.

FIG. 41A is an example of a top plan view of a pixel in the case wherean MVA mode and a transistor are combined. By applying the pixelstructure shown in FIG. 41A to a liquid crystal display device, a liquidcrystal display device having a wide viewing angle, high response speed,and high contrast can be obtained.

The pixel shown in FIG. 41A includes a scan line 10501, a video signalline 10502, a capacitor line 10503, a transistor 10504, a pixelelectrode 10505, a pixel capacitor 10506, and an alignment controlprojection 10507.

The scan line 10501 has a function of transmitting a signal (a scansignal) to the pixel. The image signal line 10502 has a function fortransmitting a signal (an image signal) to the pixel. Note that sincethe scan line 10501 and the image signal line 10502 are arranged inmatrix, they are formed of conductive layers in different layers. Notealso that a semiconductor layer may be provided at an intersection ofthe scan line 10501 and the image signal line 10502. Thus, intersectioncapacitance between the scan line 10501 and the image signal line 10502can be reduced.

The capacitor line 10503 is provided in parallel to the pixel electrode10505. A portion where the capacitor line 10503 and the pixel electrode10505 overlap with each other corresponds to the pixel capacitor 10506.Note that part of the capacitor line 10503 is extended along the imagesignal line 10502 so as to surround the image signal line 10502. Thus,crosstalk can be reduced. Crosstalk is a phenomenon in which a potentialof an electrode, which should hold the potential, is changed inaccordance with change in potential of the image signal line 10502. Notealso that intersection capacitance can be reduced by providing asemiconductor layer between the capacitor line 10503 and the imagesignal line 10502. Note also that the capacitor line 10503 is formed ofa material which is similar to that of the scan line 10501.

The transistor 10504 has a function as a switch which turns on the imagesignal line 10502 and the pixel electrode 10505. Note that one of asource region and a drain region of the transistor 10504 is provided soas to be surrounded by the other of the source region and the drainregion of the transistor 10504. Thus, the channel width of thetransistor 10504 increases, so that switching capability can beimproved. Note also that a gate electrode of the transistor 10504 isprovided so as to surround the semiconductor layer.

The pixel electrode 10505 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10504. The pixelelectrode 10505 is an electrode for applying signal voltage which istransmitted by the image signal line 10502 to a liquid crystal element.Note that the pixel electrode 10505 is rectangular. Thus, an apertureratio can be improved. Note also that as the pixel electrode 10505, alight-transmitting material may be used or a reflective material may beused. Alternatively, the pixel electrode 10505 may be formed bycombining a light-transmitting material and a reflective material.

The alignment control projection 10507 is formed on a counter substrate.The alignment control projection 10507 has a function of aligning liquidcrystal molecules radially. Note that a shape of the alignment controlprojection 10507 is not particularly limited. For example, the alignmentcontrol projection 10507 may be a dogleg shape. Thus, a plurality ofregions having different alignment of the liquid crystal molecules canbe formed, so that a viewing angle can be improved.

FIG. 41B is an example of a top plan view of a pixel in the case where aPVA mode and a transistor are combined. By applying the pixel structureshown in FIG. 41B to a liquid crystal display device, a liquid crystaldisplay device having a wide viewing angle, high response speed, andhigh contrast can be obtained.

The pixel shown in FIG. 41B includes a scan line 10511, a video signalline 10512, a capacitor line 10513, a transistor 10514, a pixelelectrode 10515, a pixel capacitor 10516, and an electrode cutoutportion 10517.

The scan line 10511 has a function of transmitting a signal (a scansignal) to the pixel. The image signal line 10512 has a function fortransmitting a signal (an image signal) to the pixel. Note that sincethe scan line 10511 and the image signal line 10512 are arranged inmatrix, they are formed of conductive layers in different layers. Notealso that a semiconductor layer may be provided at an intersection ofthe scan line 10511 and the image signal line 10512. Thus, intersectioncapacitance between the scan line 10511 and the image signal line 10512can be reduced.

The capacitor line 10513 is provided in parallel to the pixel electrode10515. A portion where the capacitor line 10513 and the pixel electrode10515 overlap with each other corresponds to the pixel capacitor 10516.Note that part of the capacitor line 10513 is extended along the imagesignal line 10512 so as to surround the image signal line 10512. Thus,crosstalk can be reduced. Crosstalk is a phenomenon in which a potentialof an electrode, which should hold the potential, is changed inaccordance with change in potential of the image signal line 10512. Notealso that intersection capacitance can be reduced by providing asemiconductor layer between the capacitor line 10513 and the imagesignal line 10512. Note also that the capacitor line 10513 is formed ofa material which is similar to that of the scan line 10511.

The transistor 10514 has a function as a switch which turns on the imagesignal line 10512 and the pixel electrode 10515. Note that one of asource region and a drain region of the transistor 10514 is provided soas to be surrounded by the other of the source region and the drainregion of the transistor 10514. Thus, the channel width of thetransistor 10514 increases, so that switching capability can beimproved. Note also that a gate electrode of the transistor 10514 isprovided so as to surround the semiconductor layer.

The pixel electrode 10515 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10514. The pixelelectrode 10515 is an electrode for applying signal voltage which istransmitted by the image signal line 10512 to a liquid crystal element.Note that the pixel electrode 10515 has a shape which is formed inaccordance with a shape of the electrode cutout portion 10517.Specifically, the pixel electrode 10515 has a shape in which a portionwhere the pixel electrode 10515 is cut is formed in a portion where theelectrode cutout portion 10517 is not formed. Thus, since a plurality ofregions having different alignment of the liquid crystal molecules canbe formed, a viewing angle can be improved. Note also that as the pixelelectrode 10515, a light-transmitting material or a reflective materialmay be used. Alternatively, the pixel electrode 10515 may be formed bycombining a light-transmitting material and a reflective material.

FIG. 42A is an example of a top plan view of a pixel in the case wherean IPS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 42A to a liquid crystal display device, a liquidcrystal display device theoretically having a wide viewing angle andresponse speed which has low dependency on a gray scale can be obtained.

The pixel shown in FIG. 42A includes a scan line 10601, a video signalline 10602, a common electrode 10603, a transistor 10604, and a pixelelectrode 10605.

The scan line 10601 has a function of transmitting a signal (a scansignal) to the pixel. The image signal line 10602 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan line 10601 and the image signal line 10602 are arranged inmatrix, they are formed of conductive layers in different layers. Notealso that a semiconductor layer may be provided at an intersection ofthe scan line 10601 and the image signal line 10602. Thus, intersectioncapacitance between the scan line 10601 and the image signal line 10602can be reduced. Note also that the image signal line 10602 is formed inaccordance with a shape of the pixel electrode 10605.

The common electrode 10603 is provided in parallel to the pixelelectrode 10605. The common electrode 10603 is an electrode forgenerating a lateral electric field. Note that the common electrode10603 is bent comb-shaped. Note also that part of the common electrode10603 is extended along the image signal line 10602 so as to surroundthe image signal line 10602. Thus, crosstalk can be reduced. Crosstalkis a phenomenon in which a potential of an electrode, which should holdthe potential, is changed in accordance with change in potential of theimage signal line 10602. Note also that intersection capacitance can bereduced by providing a semiconductor layer between the common electrode10603 and the image signal line 10602. Part of the common electrode10603, which is provided in parallel to the scan line 10601, is formedof a material which is similar to that of the scan line 10601. Part ofthe common electrode 10603, which is provided in parallel to the pixelelectrode 10605, is formed of a material which is similar to that of thepixel electrode 10605.

The transistor 10604 has a function as a switch which turns on the imagesignal line 10602 and the pixel electrode 10605. Note that one of asource region and a drain region of the transistor 10604 is provided soas to be surrounded by the other of the source region and the drainregion of the transistor 10604. Thus, the channel width of thetransistor 10604 increases, so that switching capability can beimproved. Note also that a gate electrode of the transistor 10604 isprovided so as to surround the semiconductor layer.

The pixel electrode 10605 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10604. The pixelelectrode 10605 is an electrode for applying signal voltage which istransmitted by the image signal line 10602 to a liquid crystal element.Note that the pixel electrode 10605 is bent comb-shaped. Thus, a lateralelectric field can be applied to liquid crystal molecules. In addition,since a plurality of regions having different alignment of the liquidcrystal molecules can be formed, a viewing angle can be improved. Notealso that as the pixel electrode 10605, a light-transmitting material ora reflective material may be used. Alternatively, the pixel electrode10605 may be formed by combining a light-transmitting material and areflective material.

Note that a comb-shaped portion in the common electrode 10603 and thepixel electrode 10605 may be formed of different conductive layers. Forexample, the comb-shaped portion in the common electrode 10603 may beformed of a conductive layer which is the same as that of the scan line10601 or the image signal line 10602. Similarly, the pixel electrode10605 may be formed of a conductive layer which is the same as that ofthe scan line 10601 or the image signal line 10602.

FIG. 42B is an example of a top plan view of a pixel in the case wherean PPS mode and a transistor are combined. By applying the pixelstructure shown in FIG. 42B to a liquid crystal display device, a liquidcrystal display device theoretically having a wide viewing angle andresponse speed which has low dependency on a gray scale can be obtained.

The pixel shown in FIG. 42B includes a scan line 10611, a video signalline 10612, a common electrode 10613, a transistor 10614, and a pixelelectrode 10615.

The scan line 10611 has a function of transmitting a signal (a scansignal) to the pixel. The image signal line 10612 has a function oftransmitting a signal (an image signal) to the pixel. Note that sincethe scan line 10611 and the image signal line 10612 are arranged inmatrix, they are formed of conductive layers in different layers. Notealso that a semiconductor layer may be provided at an intersection ofthe scan line 10611 and the image signal line 10612. Thus, intersectioncapacitance between the scan line 10611 and the image signal line 10612can be reduced. Note also that the image signal line 10612 is formed inaccordance with a shape of the pixel electrode 10615.

The common electrode 10613 is formed uniformly below the pixel electrode10615 and below and between the pixel electrodes 10615. Note that as thecommon electrode 10613, a light-transmitting material or a reflectivematerial may be used. Alternatively, the common electrode 10613 may beformed by combining a material in which a light-transmitting materialand a reflective material.

The transistor 10614 has a function as a switch which turns on the imagesignal line 10612 and the pixel electrode 10615. Note that one of asource region and a drain region of the transistor 10614 is provided soas to be surrounded by the other of the source region and the drainregion of the transistor 10614. Thus, the channel width of thetransistor 10614 increases, so that switching capability can beimproved. Note also that a gate electrode of the transistor 10614 isprovided so as to surround the semiconductor layer.

The pixel electrode 10615 is electrically connected to one of a sourceelectrode and a drain electrode of the transistor 10614. The pixelelectrode 10615 is an electrode for applying signal voltage which istransmitted by the image signal line 10612 to a liquid crystal element.Note that the pixel electrode 10615 is bent comb-shaped. The comb-shapedpixel electrode 10615 is provided to be closer to a liquid crystal layerthan a uniform portion of the common electrode 10613. Thus, a lateralelectric field can be applied to liquid crystal molecules. In addition,a plurality of regions having different alignment of the liquid crystalmolecules can be formed, so that a viewing angle can be improved. Notealso that as the pixel electrode 10615, a light-transmitting material ora reflective material may be used. Alternatively, the pixel electrode10615 may be formed by combining a light-transmitting material and areflective material.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 6

In this embodiment mode, a peripheral portion of a liquid crystal panelis described.

FIG. 43 is a cross-sectional view showing an example of a liquid crystaldisplay device including a so-called edge-light type backlight unit20101 and a liquid crystal panel 20107. An edge-light type correspondsto a type in which a light source is provided at an end of a backlightunit and fluorescence of the light source is emitted from the entirelight-emitting surface. The edge-light type backlight unit is thin andcan save power.

The backlight unit 20101 includes a diffusion plate 20102, a light guideplate 20103, a reflection plate 20104, a lamp reflector 20105, and alight source 20106.

The light source 20106 has a function of emitting light as necessary.For example, as the light source 20106, a cold cathode fluorescent lamp,a hot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like is used. The lamp reflector20105 has a function of efficiently guiding fluorescence from the lightsource 20106 to the light guide plate 20103. The light guide plate 20103has a function of guiding light to the entire surface by totalreflection of fluorescence. The diffusion plate 20102 has a function ofreducing variations in brightness. The reflection plate 20104 has afunction of reflecting light which is leaked from the light guide plate20103 downward (a direction which is opposite to the liquid crystalpanel 20107) to be reused.

A control circuit for controlling luminance of the light source 20106 isconnected to the backlight unit 20101. By using this control circuit,luminance of the light source 20106 can be controlled.

FIGS. 44A to 44D are views each showing a detailed structure of theedge-light type backlight unit. Note that description of a diffusionplate, a light guide plate, a reflection plate, and the like is omitted.

A backlight unit 20201 shown in FIG. 44A has a structure in which a coldcathode fluorescent lamp 20203 is used as a light source. In addition, alamp reflector 20202 is provided to efficiently reflect light from thecold cathode fluorescent lamp 20203. Such a structure is often used fora large display device because luminance from the cold cathodefluorescent lamp is high.

A backlight unit 20211 shown in FIG. 44B has a structure in whichlight-emitting diodes (LEDs) 20213 are used as light sources. Forexample, the light-emitting diodes (LEDs) 20213 which emit white lightare provided at a predetermined interval. In addition, a lamp reflector20212 is provided to efficiently reflect light from the light-emittingdiodes (LEDs) 20213.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes is suitable for a large display device. Inaddition, since light-emitting diodes are excellent in colorreproductivity, an arrangement area can be reduced. Therefore, a frameof a display device can be narrowed.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes, color reproductivity can be improved.

A backlight unit 20221 shown in FIG. 44C has a structure in whichlight-emitting diodes (LEDs) 20223, light-emitting diodes (LEDs) 20224,and light-emitting diodes (LEDs) 20225 of R, G, and B are used as lightsources. The light-emitting diodes (LEDs) 20223, the light-emittingdiodes (LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, Crand B are each provided at a predetermined interval. By using thelight-emitting diodes (LEDs) 20223, the light-emitting diodes (LEDs)20224, and the light-emitting diodes (LEDs) 20225 of R, Q, and B, colorreproductivity can be improved. In addition, a lamp reflector 20222 isprovided to efficiently reflect light from the light-emitting diodes.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes is suitable for a large display device. Inaddition, since light-emitting diodes are excellent in colorreproductivity, an arrangement area can be reduced. Therefore, a frameof a display device can be narrowed.

By sequentially making the light-emitting diodes of R, G, and B emitlight in accordance with time, color display can be performed. This is aso-called field sequential mode.

In addition, a light-emitting diode which emits white light can becombined with the light-emitting diodes (LEDs) 20223, the light-emittingdiodes (LEDs) 20224, and the light-emitting diodes (LEDs) 20225 of R, G,and B.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes, color reproductivity can be improved.

A backlight unit 20231 shown in FIG. 44D has a structure in whichlight-emitting diodes (LEDs) 20233, light-emitting diodes (LEDs) 20234,and light-emitting diodes (LEDs) 20235 of R, and B are used as lightsources. For example, among the light-emitting diodes (LEDs) 20233, thelight-emitting diodes (LEDs) 20234, and the light-emitting diodes (LEDs)20235 of R, G, and B, the light-emitting diodes of a color with lowemission intensity (e.g., green) are provided more than otherlight-emitting diodes. By using the light-emitting diodes (LEDs) 20233,the light-emitting diodes (LEDs) 20234, and the light-emitting diodes(LEDs) 20235 of R, G, and B, color reproductivity can be improved. Inaddition, a lamp reflector 20232 is provided to efficiently reflectlight from the light-emitting diodes.

Since luminance of light-emitting diodes is high, a structure usinglight-emitting diodes is suitable for a large display device. Inaddition, since light-emitting diodes are excellent in colorreproductivity, an arrangement area can be reduced. Therefore, a frameof a display device can be narrowed.

By sequentially making the light-emitting diodes of R, and B emit lightin accordance with time, color display can be performed. This is aso-called field sequential mode.

In addition, a light-emitting diode which emits white light can becombined with the light-emitting diodes (LEDs) 20233, the light-emittingdiodes (LEDs) 20234, and the light-emitting diodes (LEDs) 20235 of R,and B.

Note that in the case where light-emitting diodes are mounted on a largedisplay device, the light-emitting diodes can be provided on a back sideof the substrate. The light-emitting diodes of R, G, and B aresequentially provided at a predetermined interval. By providing thelight-emitting diodes, color reproductivity can be improved.

FIG. 47A is a cross-sectional view showing an example of a liquidcrystal display device including a so-called direct-type backlight unitand a liquid crystal panel. A direct type corresponds to a type in whicha light source is provided directly under a light-emitting surface andfluorescence of the light source is emitted from the entirelight-emitting surface. The direct-type backlight unit can efficientlyutilize the amount of emitted light.

A backlight unit 20500 includes a diffusion plate 20501, alight-shielding plate 20502, a lamp reflector 20503, and a light source20504. In addition, a reference numeral 20505 denotes a liquid crystalpanel.

The light source 20504 has a function of emitting light as necessary.For example, as the light source 20505, a cold cathode fluorescent lamp,a hot cathode fluorescent lamp, a light-emitting diode, an inorganic ELelement, an organic EL element, or the like is used. The lamp reflector20503 has a function of efficiently guiding fluorescence from the lightsource 20504 to the diffusion plate 20501 and the light-shielding plate20502. The light-shielding plate 20502 has a function of reducingvariations in brightness by shielding much light as light becomesintense in accordance with provision of the light source 20504. Thediffusion plate 20501 also has a function of reducing variations inbrightness.

A control circuit for controlling luminance of the light source 20504 isconnected to the backlight unit 20501. By using this control circuit,luminance of the light source 20504 can be controlled.

FIG. 47B is also a cross-sectional view showing an example of a liquidcrystal display device including a so-called direct-type backlight unitand a liquid crystal panel.

A backlight unit 20510 includes a diffusion plate 20511; alight-shielding plate 20512; a lamp reflector 20513; and a light source(R) 20514 a, a light source (G) 20514 b, and a light source (B) 20514 cof R, G, and B. In addition, a reference numeral 20515 denotes a liquidcrystal panel.

Each of the light source (R) 20514 a, the light source (G) 20514 b, andthe light source (B) 20514 c of R, and B has a function of emittinglight as necessary. For example, as each of the light source (R) 20514a, the light source (G) 20514 b, and the light source (B) 20514 c, acold cathode fluorescent lamp, a hot cathode fluorescent lamp, alight-emitting diode, an inorganic EL element, an organic EL element, orthe like is used. The lamp reflector 20513 has a function of efficientlyguiding fluorescence from the light sources 20514 a to 20514 c to thediffusion plate 20511 and the light-shielding plate 20512. Thelight-shielding plate 20512 has a function of reducing variations inbrightness by shielding much light as light becomes intense inaccordance with provision of the light sources 20514 a to 20514 c. Thediffusion plate 20511 also has a function of reducing variations inbrightness.

A control circuit for controlling luminance of the light source (R)20514 a, the light source (G) 20514 b, and the light source (B) 20514 cof R, and B is connected to the backlight unit 20511. By using thiscontrol circuit, luminance of the light source (R) 20514 a, the lightsource (G) 20514 b, and the light source (B) 20514 c of R, G, and B canbe controlled.

FIG. 45 is a cross-sectional view showing an example of a structure of apolarizing plate (also referred to as a polarizing film).

A polarizing film 20300 includes a protective film 20301, a substratefilm 20302, a PVA polarizing film 20303, a substrate film 20304, anadhesive layer 20305, and a mold release film 20306.

The PVA polarizing film 20303 has a function of generating light in onlya certain vibration direction (linear polarized light). Specifically,the PVA polarizing film 20303 includes molecules (polarizers) in whichlengthwise electron density and widthwise electron density are greatlydifferent from each other. The PVA polarizing film 20303 can generatelinear polarized light by uniforming directions of the molecules inwhich lengthwise electron density and widthwise electron density aregreatly different from each other.

For example, a high molecular film of polyvinyl alcohol is doped with aniodine compound and a PVA film is pulled in a certain direction, so thata film in which iodine molecules are aligned in a certain direction canbe obtained as the PVA polarizing film 20303. In this film, light whichis parallel to a major axis of the iodine molecule is absorbed by theiodine molecule. Note that a dichroism dye may be used instead of iodinefor high durability use and high heat resistance use. Note also that itis preferable that the dye be used for a liquid crystal display devicewhich needs to have durability and heat resistance, such as an in-carLCD or an LCD for a projector.

When the PVA polarizing film 20303 is sandwiched by films to be basematerials (the substrate film 20302 and the substrate film 20304) fromboth sides, reliability can be improved. Note that the PVA polarizingfilm 20303 may be sandwiched by triacetylcellulose (TAC) films with hightransparency and high durability. Note also that each of the substratefilms and the TAC films function as protective films of polarizerincluded in the PVA polarizing film 20303.

The adhesive layer 20305 which is to be attached to a glass substrate ofthe liquid crystal panel is attached to one of the substrate films (thesubstrate film 20304). Note that the adhesive layer 20305 is formed byapplying an adhesive to one of the substrate films (the substrate film20304). The mold release film 20306 (a separate film) is provided to theadhesive layer 20305.

The protective film 20301 is provided to the other one of the substratesfilms (the substrate film 20302).

A hard coating scattering layer (an anti-glare layer) may be provided ona surface of the polarizing film 20300. Since the surface of the hardcoating scattering layer has minute unevenness formed by AG treatmentand has an anti-glare function which scatters external light, reflectionof external light in the liquid crystal panel can be prevented. Surfacereflection can also be prevented.

Note also that a treatment in which plurality of optical thin filmlayers having different refractive indexes are layered (also referred toas anti-reflection treatment or AR treatment) may be performed on thesurface of the polarizing film 20300. The plurality of layered opticalthin film layers having different refractive indexes can reducereflectivity on the surface by an interference effect of light.

FIGS. 46A to 46C are diagrams each showing an example of a system blockof the liquid crystal display device.

In a pixel portion 20405, signal lines 20412 which are extended from asignal line driver circuit 20403 are provided. In addition, in the pixelportion 20405, scan lines 20410 which are extended from a scan linedriver circuit 20404 are also provided. In addition, a plurality ofpixels are arranged in matrix in cross regions of the signal lines 20412and the scan lines 20410. Note that each of the plurality of pixelsincludes a switching element. Therefore, voltage for controllinginclination of liquid crystal molecules can be separately input to eachof the plurality of pixels. A structure in which a switching element isprovided in each cross region in this manner is referred to as an activematrix type. Note also that the present invention is not limited to suchan active matrix type and a structure of a passive matrix type may beused. Since the passive matrix type does not have a switching element ineach pixel, a process is simple.

A driver circuit portion 20408 includes a control circuit 20402, thesignal line driver circuit 20403, and the scan line driver circuit20404. An image signal 20401 is input to the control circuit 20402. Thesignal line driver circuit 20403 and the scan line driver circuit 20404are controlled by the control circuit 20402 in accordance with thisimage signal 20401. That is, the control circuit 20402 inputs a controlsignal to each of the signal line driver circuit 20403 and the scan linedriver circuit 20404. Then, in accordance with this control signal, thesignal line driver circuit 20403 inputs a video signal to each of thesignal lines 20412 and the scan line driver circuit 20404 inputs a scansignal to each of the scan lines 20410. Then, the switching elementincluded in the pixel is selected in accordance with the scan signal andthe video signal is input to a pixel electrode of the pixel.

Note that the control circuit 20402 also controls a power source 20407in accordance with the image signal 20401. The power source 20407includes a unit for supplying power to a lighting unit 20406. As thelighting unit 20406, an edge-light type backlight unit or a direct-typebacklight unit can be used. Note also that a front light may be used asthe lighting unit 20406. A front light corresponds to a plate-likelighting unit including a luminous body and a light conducting body,which is attached to the front surface side of a pixel portion andilluminates the whole area. By using such a lighting unit, the pixelportion can be uniformly illuminated at low power consumption.

As shown in FIG. 46B, the scan line driver circuit 20404 includes ashift register 20441, a level shifter 20442, and a circuit functioningas a buffer 20443. A signal such as a gate start pulse (GSP) or a gateclock signal (GCK) is input to the shift register 20441.

As shown in FIG. 46C, the signal line driver circuit 20403 includes ashift register 20431, a first latch 20432, a second latch 20433, a levelshifter 20434, and a circuit functioning as a buffer 20435. The circuitfunctioning as the buffer 20435 corresponds to a circuit which has afunction of amplifying a weak signal and includes an operationalamplifier or the like. A signal such as a start pulse (SSP) is input tothe level shifter 20434 and data (DATA) such as a video signal is inputto the first latch 20432. A latch (LAT) signal can be temporally held inthe second latch 20433 and is simultaneously input to the pixel portion20405. This is referred to as line sequential driving. Therefore, when apixel is used in which not line sequential driving but dot sequentialdriving is performed, the second latch can be omitted.

Note that in this embodiment mode, a known liquid crystal panel can beused for the liquid crystal panel. For example, a structure in which aliquid crystal layer is sealed between two substrates can be used as theliquid crystal panel. A transistor, a capacitor, a pixel electrode, analignment film, or the like is formed over one of the substrates. Apolarizing plate, a retardation plate, or a prism sheet may be providedon the surface opposite to a top surface of the one of the substrates. Acolor filter, a black matrix, a counter electrode, an alignment film, orthe like is provided on the other one of the substrates. A polarizingplate or a retardation plate may be provided on the surface opposite toa top surface of the other one of the substrates. The color filter andthe black matrix may be formed over the top surface of the one of thesubstrates. Note also that three-dimensional display can be performed byproviding a slit (a grid) on the top surface side of the one of thesubstrates or the surface opposite to the top surface side of the one ofthe substrates.

Each of the polarizing plate, the retardation plate, and the prism sheetcan be provided between the two substrates. Alternatively, each of thepolarizing plate, the retardation plate, and the prism sheet can beintegrated with one of the two substrates.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 7

In this embodiment mode, a driving method of a display device isdescribed. In particular, a driving method of a liquid crystal displaydevice is described.

First, overdriving is described with reference to FIGS. 48A to 48C. FIG.48A shows time change in output luminance of a display element withrespect to input voltage. Time change in output luminance of the displayelement with respect to input voltage 30121 represented by a dashed line30121 is as shown by output luminance 30123 represented by a dashed linesimilarly. That is, although voltage for obtaining intended outputluminance Lo is Vi, time in accordance with response speed of theelement is necessary before output luminance reaches the intended outputluminance Lo when Vi is directly input as input voltage.

Overdriving is a technique for increasing this response speed.Specifically, this is a method as follows: first, Vo which is largervoltage than Vi is applied to the element for a certain time to increaseresponse speed of the element and output luminance is made close to theintended output luminance Lo, and then, the input voltage is returned toVi. The input voltage and the output luminance at this time are as shownby input voltage 30122 and output voltage 30124, respectively. In thegraph of the output luminance 30124, time for reaching the intendedoutput luminance Lo is shorter than that of the output luminance 30123.

Note that although the case where output luminance is changed positivelywith respect to input voltage is described in FIG. 48A, this embodimentmode also includes the case where output luminance is changed negativelywith respect to input voltage.

A circuit for realizing such driving is described with reference toFIGS. 48B and 48C. First, the case where an input image signal 30131 isa signal having an analog value (may be a discrete value) and an outputimage signal 30132 is also a signal having an analog value is describedwith reference to FIG. 48B. An overdriving circuit shown in FIG. 48Bincludes an encoding circuit 30101, a frame memory 30102, a correctioncircuit 30103, and a D/A converter circuit 30104.

First, the input image signal 30131 is input to the encoding circuit30101 and encoded. That is, the input image signal 30131 is convertedfrom an analog signal into a digital signal with an appropriate bitnumber. After that, the converted digital signal is input to each of theframe memory 30102 and the correction circuit 30103. An image signal ofthe previous frame which is held in the frame memory 30102 is input tothe correction circuit 30103 at the same time. Then, in the correctioncircuit 30103, an image signal corrected using an image signal of aframe and the image signal of the previous frame is output in accordancewith a numeric value table which is prepared in advance. At this time,an output switching signal 30133 may be input to the correction circuit30103 and the corrected image signal and the image signal of the framemay be switched to be output. Next, the corrected image signal or theimage signal of the frame is input to the D/A converter circuit 30104.Then, the output image signal 30132 which is an analog signal having avalue in accordance with the corrected image signal or the image signalof the frame is output. In this manner, overdriving is realized.

Next, the case where the input image signal 30131 is a signal having adigital value and the output image signal 30132 is also a signal havinga digital value is described with reference to FIG. 48C. An overdrivingcircuit shown in FIG. 48C includes a frame memory 30112 and a correctioncircuit 30113.

First, the input image signal 30131 is a digital signal and is input toeach of the frame memory 30112 and the correction circuit 30113. Animage signal of the previous frame which is held in the frame memory30112 is input to the correction circuit 30113 at the same time. Then,in the correction circuit 30113, an image signal corrected using animage signal of a frame and the image signal of the previous frame isoutput in accordance with a numeric value table which is prepared inadvance. At this time, the output switching signal 30133 may be input tothe correction circuit 30113 and the corrected image signal and theimage signal of the frame may be switched to be output. In this manner,overdriving is realized.

Note that the case where the input image signal 30131 is an analogsignal and the output image signal 30132 is a digital signal is includedin the overdriving circuit in this embodiment mode. At this time, it isonly necessary to omit the D/A converter circuit 30104 from the circuitshown in FIG. 48B. In addition, the case where the input image signal30131 is a digital signal and the output image signal 30132 is an analogsignal is included in the overdriving circuit in this embodiment mode.At this time, it is only necessary to omit the encoding circuit 30101from the circuit shown in FIG. 48B.

Driving which controls a potential of a common line is described withreference to FIGS. 49A and 49B. FIG. 49A is a diagram showing aplurality of pixel circuits in which one common line is provided withrespect to one scan line in a display device using a display elementwhich has capacitive properties like a liquid crystal element. Each ofthe pixel circuits shown in FIG. 49A includes a transistor 30201, anauxiliary capacitor 30202, a display element 30203, a video signal line30204, a scan line 30205, and a common line 30206.

A gate electrode of the transistor 30201 is electrically connected tothe scan line 30205; one of a source electrode and a drain electrode ofthe transistor 30201 is electrically connected to the video signal line30204; and the other of the source electrode and the drain electrode ofthe transistor 30201 is electrically connected to one of electrodes ofthe auxiliary capacitor 30202 and one of electrodes of the displayelement 30203. In addition, the other of the electrodes of the auxiliarycapacitor 30202 is electrically connected to the common line 30206.

First, in each of pixels selected by the scan line 30205, voltagecorresponding to an image signal is applied to the display element 30203and the auxiliary capacitor 30202 through the video signal line 30204because the transistor 30201 is turned on. At this time, when the imagesignal is a signal which makes all of pixels connected to the commonline 30206 display a minimum gray scale or when the image signal is asignal which makes all of the pixels connected to the common line 30206display a maximum gray scale, it is not necessary that the image signalbe written in each of the pixels through the video signal line 30204.Voltage applied to the display element 30203 can be changed by changinga potential of the common line 30206 instead of writing the image signalthrough the video signal line 30204.

Next, FIG. 49B is a diagram showing a plurality of pixel circuits inwhich two common lines are provided with respect to one scan line in adisplay device using a display element which has capacitive propertieslike a liquid crystal element. Each of the pixel circuits shown in FIG.49B includes a transistor 30211, an auxiliary capacitor 30212, a displayelement 30213, a video signal line 30214, a scan line 30215, a firstcommon line 30216, and a second common line 30217.

A gate electrode of the transistor 30211 is electrically connected tothe scan line 30215; one of a source electrode and a drain electrode ofthe transistor 30211 is electrically connected to the video signal line30214; and the other of the source electrode and the drain electrode ofthe transistor 30211 is electrically connected to one of electrodes ofthe auxiliary capacitor 30212 and one of electrodes of the displayelement 30213. In addition, the other of the electrodes of the auxiliarycapacitor 30212 is electrically connected to the first common line30216. Further, in a pixel which is adjacent to the pixel, the other ofthe electrodes of the auxiliary capacitor 30212 is electricallyconnected to the second common line 30217.

In the pixel circuits shown in FIG. 49B, the number of pixels which areelectrically connected to one common line is small. Therefore, bychanging a potential of the first common line 30216 or the second commonline 30217 instead of writing an image signal through the video signalline 30214, frequency of changing voltage applied to the display element30213 is significantly increased. In addition, source inversion drivingor dot inversion driving can be performed. By performing sourceinversion driving or dot inversion driving, reliability of the elementcan be improved and a flicker can be suppressed.

A scanning backlight is described with reference to FIGS. 50A to 50C.FIG. 50A is a view showing a scanning backlight in which cold cathodefluorescent lamps are arranged. The scanning backlight shown in FIG. 50Aincludes a diffusion plate 30301 and N pieces of cold cathodefluorescent lamps 30302-1 to 30302-N. The N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N are arranged on the back side ofthe diffusion plate 30301, so that the N pieces of the cold cathodefluorescent lamps 30302-1 to 30302-N can be scanned while luminancethereof is changed.

Change in luminance of each of the cold cathode fluorescent lamps inscanning is described with reference to FIG. 50C. First, luminance ofthe cold cathode fluorescent lamp 30302-1 is changed for a certainperiod. After that, luminance of the cold cathode fluorescent lamp30302-2 which is provided adjacent to the cold cathode fluorescent lamp30302-1 is changed for the same period. In this manner, luminance ischanged sequentially from the cold cathode fluorescent lamp 30302-1 tothe cold cathode fluorescent lamp 30302-N. Although luminance which ischanged for a certain period is set to be lower than original luminancein FIG. 50C, it may also be higher than original luminance. In addition,although scanning is performed from the cold cathode fluorescent lamps30302-1 to 30302-N, scanning may also be performed from the cold cathodefluorescent lamps 30302-N to 30302-1, which is in a reversed order.

By performing driving as in FIG. 50C, average luminance of the backlightcan be decreased. Therefore, power consumption of the backlight, whichmainly takes up power consumption of the liquid crystal display device,can be reduced.

Note that an LED may be used as a light source of the scanningbacklight. The scanning backlight in that case is as shown in FIG. 50B.The scanning backlight shown in FIG. 50B includes a diffusion plate30311 and light sources 30312-1 to 30312-N, in each of which LEDs arearranged. When the LED is used as the light source of the scanningbacklight, there is an advantage in that the backlight can be thin andlightweight. In addition, there is also an advantage that a colorreproduction area can be widened. Further, since the LEDs which arearranged in each of the light sources 303124 to 30312-N can be similarlyscanned, a dot scanning backlight can also be obtained. By using the dotscanning backlight, quality of a moving image can be further improved.

Note that when the LED is used as the light source of the backlight,driving can be performed by changing luminance as shown in FIG. 50C.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 8

In this embodiment mode, a pixel structure and an operation of a pixelwhich can be applied to a liquid crystal display device are described.

In this embodiment mode, as an operation mode of a liquid crystalelement, a TN (twisted nematic) mode, an IPS (in-plane-switching) mode,an FFS (fringe field switching) mode, an MVA (multi-domain verticalalignment) mode, a PVA (patterned vertical alignment) mode, an ASM(axially symmetric aligned micro-cell) mode, an OCB (optical compensatedbirefringence) mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(antiferroelectric liquid crystal) mode, or the like can be used.

FIG. 51A is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device.

A pixel 40100 includes a transistor 40101, a liquid crystal element40102, and a capacitor 40103. A gate electrode of the transistor 40101is connected to a wiring 40105. A first electrode of the transistor40101 is connected to a wiring 40104. A second electrode of thetransistor 40101 is connected to a first electrode of the liquid crystalelement 40102 and a first electrode of the capacitor 40103. A secondelectrode of the liquid crystal element 40102 corresponds to a counterelectrode 40107. A second electrode of the capacitor 40103 is connectedto a wiring 40106.

The wiring 40104 functions as a signal line. The wiring 40105 functionsas a scan line. The wiring 40106 functions as a capacitor line. Thetransistor 40101 functions as a switch. The capacitor 40103 functions asa storage capacitor.

It is only necessary that the transistor 40101 function as a switch, andthe transistor 40101 may be a P-channel transistor or an N-channeltransistor.

A video signal is input to the wiring 40104. A scan signal is input tothe wiring 40105. A constant potential is supplied to the wiring 40106.Note that the scan signal is an H-level or L-level digital voltagesignal. In the case where the transistor 40101 is an N-channeltransistor, an H level of the scan signal is a potential which can turnon the transistor 40101 and an L level of the scan signal is a potentialwhich can turn off the transistor 40101. Alternatively, in the casewhere the transistor 40101 is a P-channel transistor, the H level of thescan signal is a potential which can turn off the transistor 40101 andthe L level of the scan signal is a potential which can turn on thetransistor 40101. Note that the video signal has analog voltage. Thevideo signal is a potential which is lower than the H level of the scansignal and higher than the L level of the scan signal. Note also thatthe constant potential supplied to the wiring 40106 is preferably equalto a potential of the counter electrode 40107.

Operations of the pixel 40100 are described by diving the wholeoperations into the case where the transistor 40101 is on and the casewhere the transistor 40101 is off.

In the case where the transistor 40101 is on, the wiring 40104 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40102 and the first electrode of the capacitor40103. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40102 and the firstelectrode of the capacitor 40103 from the wiring 40104 through thetransistor 40101. In addition, the capacitor 40103 holds a potentialdifference between a potential of the video signal and the potentialsupplied to the wiring 40106.

In the case where the transistor 40101 is off, the wiring 40104 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40102 and the first electrode of thecapacitor 40103. Therefore, each of the first electrode of the liquidcrystal element 40102 and the first electrode of the capacitor 40103 isset in a floating state. Since the capacitor 40103 holds the potentialdifference between the potential of the video signal and the potentialsupplied to the wiring 40106, each of the first electrode of the liquidcrystal element 40102 and the first electrode of the capacitor 40103holds a potential which is the same as or corresponds to the videosignal. Note that the liquid crystal element 40102 has transmittance inaccordance with the video signal.

FIG. 51B is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device. In particular, FIG. 51Bis a diagram showing an example of a pixel structure which can beapplied to a liquid crystal display device suitable for a lateralelectric field mode (including an IPS mode and an FFS mode).

A pixel 40110 includes a transistor 40111, a liquid crystal element40112, and a capacitor 40113. A gate electrode of the transistor 40111is connected to a wiring 40115. A first electrode of the transistor40111 is connected to a wiring 40114. A second electrode of thetransistor 40111 is connected to a first electrode of the liquid crystalelement 40112 and a first electrode of the capacitor 40113. A secondelectrode of the liquid crystal element 40112 is connected to a wiring40116. A second electrode of the capacitor 40113 is connected to thewiring 40116.

The wiring 40114 functions as a signal line. The wiring 40115 functionsas a scan line. The wiring 40116 functions as a capacitor line. Thetransistor 40111 functions as a switch. The capacitor 40113 functions asa storage capacitor.

It is only necessary that the transistor 40111 function as a switch, andthe transistor 40111 may be a P-channel transistor or an N-channeltransistor.

A video signal is input to the wiring 40114. A scan signal is input tothe wiring 40115. A constant potential is supplied to the wiring 40116.Note that the scan signal is an H-level or L-level digital voltagesignal. In the case where the transistor 40111 is an N-channeltransistor, an H level of the scan signal is a potential which can turnon the transistor 40111 and an L level of the scan signal is a potentialwhich can turn off the transistor 40111. Alternatively, in the casewhere the transistor 40111 is a P-channel transistor, the H level of thescan signal is a potential which can turn off the transistor 40111 andthe L level of the scan signal is a potential which can turn on thetransistor 40111. Note that the video signal has analog voltage. Thevideo signal is a potential which is lower than the H level of the scansignal and higher than the L level of the scan signal.

Operations of the pixel 40110 are described by diving the wholeoperations into the case where the transistor 40111 is on and the casewhere the transistor 40111 is off.

In the case where the transistor 40111 is on, the wiring 40114 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40112 and the first electrode of the capacitor40113. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40112 and the firstelectrode of the capacitor 40113 from the wiring 40114 through thetransistor 40111. In addition, the capacitor 40113 holds a potentialdifference between a potential of the video signal and the potentialsupplied to the wiring 40116.

In the case where the transistor 40111 is off, the wiring 40114 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40112 and the first electrode of thecapacitor 40113. Therefore, each of the first electrode of the liquidcrystal element 40112 and the first electrode of the capacitor 40113 isset in a floating state. Since the capacitor 40113 holds the potentialdifference between the potential of the video signal and the potentialsupplied to the wiring 40116, each of the first electrode of the liquidcrystal element 40112 and the first electrode of the capacitor 40113holds a potential which is the same as or corresponds to the videosignal. Note that the liquid crystal element 40112 has transmittance inaccordance with the video signal.

FIG. 52 is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device. In particular, FIG. 52is a diagram showing an example of a pixel structure in which anaperture ratio of a pixel can be increased by reducing the number ofwirings.

FIG. 52 shows two pixels which are provided in the same column direction(a pixel 40200 and a pixel 40210). For example, when the pixel 40200 isprovided in an N-th row, the pixel 40210 is provided in an (N+1)th row.

A pixel 40200 includes a transistor 40201, a liquid crystal element40202, and a capacitor 40203. A gate electrode of the transistor 40201is connected to a wiring 40205. A first electrode of the transistor40201 is connected to a wiring 40204. A second electrode of thetransistor 40201 is connected to a first electrode of the liquid crystalelement 40202 and a first electrode of the capacitor 40203. A secondelectrode of the liquid crystal element 40202 corresponds to a counterelectrode 40207. A second electrode of the capacitor 40203 is connectedto a wiring which is the same as a wiring connected to a gate electrodeof a transistor of the previous row.

A pixel 40210 includes a transistor 40211, a liquid crystal element40212, and a capacitor 40213. A gate electrode of the transistor 40211is connected to a wiring 40215. A first electrode of the transistor40211 is connected to the wiring 40204. A second electrode of thetransistor 40211 is connected to a first electrode of the liquid crystalelement 40212 and a first electrode of the capacitor 40213. A secondelectrode of the liquid crystal element 40212 corresponds to the counterelectrode 40207. A second electrode of the capacitor 40213 is connectedto the wiring which is the same as the wiring connected to the gateelectrode of the transistor of the previous row (the wiring 40205).

The wiring 40204 functions as a signal line. The wiring 40205 functionsas a scan line of the N-th row. The wiring 40205 also functions as acapacitor line of the (N+1)th row. The transistor 40201 functions as aswitch. The capacitor 40203 functions as a storage capacitor.

The wiring 40215 functions as a scan line of the (N+1)th row. The wiring40215 also functions as a capacitor line of an (N+2)th row. Thetransistor 40211 functions as a switch. The capacitor 40213 functions asa storage capacitor.

It is only necessary that each of the transistor 40201 and thetransistor 40211 function as a switch, and each of the transistor 40201and the transistor 40211 may be a P-channel transistor or an N-channeltransistor.

A video signal is input to the wiring 40204. A scan signal (of an N-throw) is input to the wiring 40205. A scan signal (of an (N+1)th row) isinput to the wiring 40215.

The scan signal is an H-level or L-level digital voltage signal. In thecase where the transistor 40201 (or the transistor 40211) is anN-channel transistor, an H level of the scan signal is a potential whichcan turn on the transistor 40201 (or the transistor 40211) and an Llevel of the scan signal is a potential which can turn off thetransistor 40201 (or the transistor 40211). Alternatively, in the casewhere the transistor 40201 (or the transistor 40211) is a P-channeltransistor, the H level of the scan signal is a potential which can turnoff the transistor 40201 (or the transistor 40211) and the L level ofthe scan signal is a potential which can turn on the transistor 40201(or the transistor 40211). Note that the video signal has analogvoltage. The video signal is a potential which is lower than the H levelof the scan signal and higher than the L level of the scan signal.

Operations of the pixel 40200 are described by diving the wholeoperations into the case where the transistor 40201 is on and the casewhere the transistor 40201 is off.

In the case where the transistor 40201 is on, the wiring 40204 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40202 and the first electrode of the capacitor40203. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40202 and the firstelectrode of the capacitor 40203 from the wiring 40204 through thetransistor 40201. In addition, the capacitor 40203 holds a potentialdifference between a potential of the video signal and a potentialsupplied to the wiring which is the same as the wiring connected to thegate electrode of the transistor of the previous row.

In the case where the transistor 40201 is off, the wiring 40204 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40202 and the first electrode of thecapacitor 40203. Therefore, each of the first electrode of the liquidcrystal element 40202 and the first electrode of the capacitor 40203 isset in a floating state. Since the capacitor 40203 holds the potentialdifference between the potential of the video signal and the potentialof the wiring which is the same as the wiring connected to the gateelectrode of the transistor of the previous row, each of the firstelectrode of the liquid crystal element 40202 and the first electrode ofthe capacitor 40203 holds a potential which is the same as orcorresponds to the video signal. Note that the liquid crystal element40202 has transmittance in accordance with the video signal.

Operations of the pixel 40210 are described by diving the wholeoperations into the case where the transistor 40211 is on and the casewhere the transistor 40211 is off.

In the case where the transistor 40211 is on, the wiring 40204 iselectrically connected to the first electrode (a pixel electrode) of theliquid crystal element 40212 and the first electrode of the capacitor40213. Therefore, the video signal is input to the first electrode (thepixel electrode) of the liquid crystal element 40212 and the firstelectrode of the capacitor 40213 from the wiring 40204 through thetransistor 40211. In addition, the capacitor 40213 holds a potentialdifference between a potential of the video signal and a potentialsupplied to a wiring which is the same as the wiring connected to thegate electrode of the transistor of the previous row (the wiring 40205).

In the case where the transistor 40211 is off, the wiring 40214 is notelectrically connected to the first electrode (the pixel electrode) ofthe liquid crystal element 40212 and the first electrode of thecapacitor 40213. Therefore, each of the first electrode of the liquidcrystal element 40212 and the first electrode of the capacitor 40213 isset in a floating state. Since the capacitor 40213 holds the potentialdifference between the potential of the video signal and the potentialof the wiring which is the same as the wiring connected to the gateelectrode of the transistor of the previous row (the wiring 40215), eachof the first electrode (the pixel electrode) of the liquid crystalelement 40212 and the first electrode of the capacitor 40213 holds apotential which is the same as or corresponds to the video signal. Notethat the liquid crystal element 40212 has transmittance in accordancewith the video signal.

FIG. 53 is a diagram showing an example of a pixel structure which canbe applied to the liquid crystal display device. In particular, FIG. 53is a diagram showing an example of a pixel structure in which a viewingangle can be improved by using a subpixel.

A pixel 40320 includes a subpixel 40300 and a subpixel 40310. Althoughthe case in which the pixel 40320 includes two subpixels is describedbelow, the pixel 40320 may include three or more subpixels.

The subpixel 40300 includes a transistor 40301, a liquid crystal element40302, and a capacitor 40303. A gate electrode of the transistor 40301is connected to a wiring 40305. A first electrode of the transistor40301 is connected to a wiring 40304. A second electrode of thetransistor 40301 is connected to a first electrode of the liquid crystalelement 40302 and a first electrode of the capacitor 40303. A secondelectrode of the liquid crystal element 40302 corresponds to a counterelectrode 40307. A second electrode of the capacitor 40303 is connectedto a wiring 40306.

The subpixel 40310 includes a transistor 40311, a liquid crystal element40312, and a capacitor 40313. A gate electrode of the transistor 40311is connected to a wiring 40315. A first electrode of the transistor40311 is connected to the wiring 40304. A second electrode of thetransistor 40311 is connected to a first electrode of the liquid crystalelement 40312 and a first electrode of the capacitor 40313. A secondelectrode of the liquid crystal element 40312 corresponds to the counterelectrode 40307. A second electrode of the capacitor 40313 is connectedto a wiring 40306.

The wiring 40304 functions as a signal line. Each of the wiring 40305and the wiring 40315 functions as a scan line. The wiring 40306functions as a capacitor line. Each of the transistor 40301 and thetransistor 40311 functions as a switch. Each of the capacitor 40303 andthe capacitor 40313 functions as a storage capacitor.

It is only necessary that each of the transistor 40301 and thetransistor 40311 function as a switch, and each of the transistor 40301and the transistor 40311 may be a P-channel transistor or an N-channeltransistor.

A video signal is input to the wiring 40304. A scan signal is input tothe wiring 40305 and the wiring 40315. A constant potential is suppliedto the wiring 40306.

The scan signal is an H-level or L-level digital voltage signal. In thecase where the transistor 40301 (or the transistor 40311) is anN-channel transistor, an H level of the scan signal is a potential whichcan turn on the transistor 40301 (or the transistor 40311) and an Llevel of the scan signal is a potential which can turn off thetransistor 40301 (or the transistor 40311). Alternatively, in the casewhere the transistor 40301 (or the transistor 40311) is a P-channeltransistor, the H level of the scan signal is a potential which can turnoff the transistor 40301 (or the transistor 40311) and the L level ofthe scan signal is a potential which can turn on the transistor 40301(or the transistor 40311). Note that the video signal has analogvoltage. The video signal is a potential which is lower than the H levelof the scan signal and higher than the L level of the scan signal. Notealso that the constant potential supplied to the wiring 40306 ispreferably equal to a potential of the counter electrode 40307.

Operations of the pixel 40320 are described by diving the wholeoperations into the case where the transistor 40301 is on and thetransistor 40311 is off, the case where the transistor 40301 is off andthe transistor 40311 is on, and the case where the transistor 40301 andthe transistor 40311 are off.

In the case where the transistor 40301 is on and the transistor 40311 isoff, the wiring 40304 is electrically connected to the first electrode(a pixel electrode) of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 in the subpixel 40300. Therefore, thevideo signal is input to the first electrode (the pixel electrode) ofthe liquid crystal element 40302 and the first electrode of thecapacitor 40303 from the wiring 40304 through the transistor 40301. Inaddition, the capacitor 40303 holds a potential difference between apotential of the video signal and a potential supplied to the wiring40306. At this time, the wiring 40304 is not electrically connected tothe first electrode (the pixel electrode) of the liquid crystal element40312 and the first electrode of the capacitor 40313 in the subpixel40310. Therefore, the video signal is not input to the subpixel 40310.

In the case where the transistor 40301 is off and the transistor 40311is on, the wiring 40304 is not electrically connected to the firstelectrode (the pixel electrode) of the liquid crystal element 40302 andthe first electrode of the capacitor 40303 in the subpixel 40300.Therefore, each of the first electrode of the liquid crystal element40302 and the first electrode of the capacitor 40303 is set in afloating state. Since the capacitor 40303 holds the potential differencebetween the potential of the video signal and the potential supplied tothe wiring 40306, each of the first electrode of the liquid crystalelement 40302 and the first electrode of the capacitor 40303 holds apotential which is the same as or corresponds to the video signal. Atthis time, the wiring 40304 is electrically connected to the firstelectrode (the pixel electrode) of the liquid crystal element 40312 andthe first electrode of the capacitor 40313 in the subpixel 40310.Therefore, the video signal is input to the first electrode (the pixelelectrode) of the liquid crystal element 40312 and the first electrodeof the capacitor 40313 from the wiring 40304 through the transistor40311. In addition, the capacitor 40313 holds a potential differencebetween a potential of the video signal and a potential supplied to thewiring 40306.

In the case where the transistor 40301 and the transistor 40311 are off,the wiring 40304 is not electrically connected to the first electrode(the pixel electrode) of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 in the subpixel 40300. Therefore, eachof the first electrode of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 is set in a floating state. Since thecapacitor 40303 holds the potential difference between the potential ofthe video signal and the potential supplied to the wiring 40306, each ofthe first electrode of the liquid crystal element 40302 and the firstelectrode of the capacitor 40303 holds a potential which is the same asor corresponds to the video signal. Note that the liquid crystal element40302 has transmittance in accordance with the video signal. At thistime, the wiring 40304 is not electrically connected to the firstelectrode (the pixel electrode) of the liquid crystal element 40312 andthe first electrode of the capacitor 40313 similarly in the subpixel40310. Therefore, each of the first electrode of the liquid crystalelement 40312 and the first electrode of the capacitor 40313 is set in afloating state. Since the capacitor 40313 holds the potential differencebetween the potential of the video signal and the potential of thewiring 40306, each of the first electrode of the liquid crystal element40312 and the first electrode of the capacitor 40313 holds a potentialwhich is the same as or corresponds to the video signal. Note that theliquid crystal element 40312 has transmittance in accordance with thevideo signal.

A video signal input to the subpixel 40300 may be a value which isdifferent from that of a video signal input to the subpixel 40310. Inthis case, the viewing angle can be widened because alignment of liquidcrystal molecules of the liquid crystal element 40302 and alignment ofliquid crystal molecules of the liquid crystal element 40312 can bevaried from each other.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 9

In this embodiment mode, various liquid crystal modes are described.

First, various liquid crystal modes are described with reference tocross-sectional views.

FIGS. 54A and 54B are schematic views of cross sections of a TN mode.

A liquid crystal layer 50100 is held between a first substrate 50101 anda second substrate 50102 which are provided so as to be opposite to eachother. A first electrode 50105 is formed on a top surface of the firstsubstrate 50101. A second electrode 50106 is formed on a top surface ofthe second substrate 50102. A first polarizing plate 50103 is providedon a surface of the first substrate 50101, which does not face theliquid crystal layer. A second polarizing plate 50104 is provided on asurface of the second substrate 50102, which does not face the liquidcrystal layer. Note that the first polarizing plate 50103 and the secondpolarizing plate 50104 are provided so as to be in a cross nicol state.

The first polarizing plate 50103 may be provided on the top surface ofthe first substrate 50101. The second polarizing plate 50104 may beprovided on the top surface of the second substrate 50102.

It is only necessary that at least one of the first electrode 50105 andthe second electrode 50106 have transparency (a transmissive orreflective liquid crystal display device). Alternatively, both the firstelectrode 50105 and the second electrode 50106 may have transparency,and part of one of the electrodes may have reflectivity (asemi-transmissive liquid crystal display device).

FIG. 54A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50105 and the second electrode50106 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned longitudinally, light emitted from abacklight is not affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 50103 and thesecond polarizing plate 50104 are provided so as to be in a cross nicolstate, light emitted from the backlight cannot pass through thesubstrate. Therefore, black display is performed.

Note that by controlling voltage applied to the first electrode 50105and the second electrode 50106, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 54B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50105 and the secondelectrode 50106. Since the liquid crystal molecules are alignedlaterally and rotated in a plane, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50103 and the second polarizing plate50104 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed. This is a so-called normally white mode.

A liquid crystal display device having a structure shown in FIG. 54A orFIG. 54B can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50101side or a second substrate 50102 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for a TN mode.

FIGS. 55A and 55B are schematic views of cross sections of a VA mode. Inthe VA mode, liquid crystal molecules are aligned such that they arevertical to a substrate when there is no electric field.

A liquid crystal layer 50200 is held between a first substrate 50201 anda second substrate 50202 which are provided so as to be opposite to eachother. A first electrode 50205 is formed on a top surface of the firstsubstrate 50201. A second electrode 50206 is formed on a top surface ofthe second substrate 50202. A first polarizing plate 50203 is providedon a surface of the first substrate 50201, which does not face theliquid crystal layer. A second polarizing plate 50204 is provided on asurface of the second substrate 50202, which does not face the liquidcrystal layer. Note that the first polarizing plate 50203 and the secondpolarizing plate 50204 are provided so as to be in a cross nicol state.

The first polarizing plate 50203 may be provided on the top surface ofthe first substrate 50201. The second polarizing plate 50204 may beprovided on the top surface of the second substrate 50202.

It is only necessary that at least one of the first electrode 50205 andthe second electrode 50206 have transparency (a transmissive orreflective liquid crystal display device). Alternatively, both the firstelectrode 50205 and the second electrode 50206 may have transparency,and part of one of the electrodes may have reflectivity (asemi-transmissive liquid crystal display device).

FIG. 55A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50205 and the second electrode50206 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned laterally, light emitted from a backlightis affected by birefringence of the liquid crystal molecules. Inaddition, since the first polarizing plate 50203 and the secondpolarizing plate 50204 are provided so as to be in a cross nicol state,light emitted from the backlight passes through the substrate.Therefore, white display is performed.

Note that by controlling voltage applied to the first electrode 50205and the second electrode 50206, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 55B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50205 and the secondelectrode 50206. Since liquid crystal molecules are alignedlongitudinally, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50203 and the second polarizing plate 50204 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 55A orFIG. 55B can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50201side or a second substrate 50202 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for a VA mode.

FIGS. 55C and 55D are schematic views of cross sections of an MVA mode.In the MVA mode, viewing angle dependency of each portion is compensatedby each other.

A liquid crystal layer 50210 is held between a first substrate 50211 anda second substrate 50212 which are provided so as to be opposite to eachother. A first electrode 50215 is formed on a top surface of the firstsubstrate 50211. A second electrode 50216 is formed on a top surface ofthe second substrate 50212. A first protrusion 50217 for controllingalignment is formed on the first electrode 50215. A second protrusion50218 for controlling alignment is formed over the second electrode50216. A first polarizing plate 50213 is provided on a surface of thefirst substrate 50211, which does not face the liquid crystal layer. Asecond polarizing plate 50214 is provided on a surface of the secondsubstrate 50212, which does not face the liquid crystal layer. Note thatthe first polarizing plate 50213 and the second polarizing plate 50214are provided so as to be in a cross nicol state.

The first polarizing plate 50213 may be provided on the top surface ofthe first substrate 50211. The second polarizing plate 50214 may beprovided on the top surface of the second substrate 50212.

It is only necessary that at least one of the first electrode 50215 andthe second electrode 50216 have transparency (a transmissive orreflective liquid crystal display device). Alternatively, both the firstelectrode 50215 and the second electrode 50216 may have transparency,and part of one of the electrodes may have reflectivity (asemi-transmissive liquid crystal display device).

FIG. 55C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50215 and the second electrode50216 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned so as to tilt toward the first protrusion50217 and the second protrusion 50218, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50213 and the second polarizing plate50214 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed.

Note that by controlling voltage applied to the first electrode 50215and the second electrode 50216, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 55D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50215 and the secondelectrode 50216. Since liquid crystal molecules are alignedlongitudinally, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50213 and the second polarizing plate 50214 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 55C orFIG. 55D can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50211side or a second substrate 50212 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for an MVA mode.

FIGS. 56A and 56B are schematic views of cross sections of an OCB mode.In the OCB mode, viewing angle dependency is low because alignment ofliquid crystal molecules in a liquid crystal layer can be opticallycompensated. This state of the liquid crystal molecules is referred toas bend alignment.

A liquid crystal layer 50300 is held between a first substrate 50301 anda second substrate 50302 which are provided so as to be opposite to eachother. A first electrode 50305 is formed on a top surface of the firstsubstrate 50301. A second electrode 50306 is formed on a top surface ofthe second substrate 50302. A first polarizing plate 50303 is providedon a surface of the first substrate 50301, which does not face theliquid crystal layer 50300. A second polarizing plate 50304 is providedon a surface of the second substrate 50302, which does not face theliquid crystal layer 50300. Note that the first polarizing plate 50303and the second polarizing plate 50304 are provided so as to be in across nicol state.

The first polarizing plate 50303 may be provided on the top surface ofthe first substrate 50301, i.e., may be provided between the firstsubstrate 50301 and the liquid crystal layer. The second polarizingplate 50304 may be provided on the top surface of the second substrate50302, i.e., may be provided between the second substrate 50302 and theliquid crystal layer.

It is only necessary that at least one of the first electrode 50305 andthe second electrode 50306 have transparency (a transmissive orreflective liquid crystal display device). Alternatively, both the firstelectrode 50305 and the second electrode 50306 may have transparency,and part of one of the electrodes may have reflectivity (asemi-transmissive liquid crystal display device).

FIG. 56A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50305 and the second electrode50306 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned longitudinally, light emitted from abacklight is not affected by birefringence of the liquid crystalmolecules. In addition, since the first polarizing plate 50303 and thesecond polarizing plate 50304 are provided so as to be in a cross nicolstate, light emitted from the backlight does not pass through thesubstrate. Therefore, black display is performed.

Note that by controlling voltage applied to the first electrode 50305and the second electrode 50306, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 56B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50305 and the secondelectrode 50306. Since liquid crystal molecules are in a bend alignmentstate, light emitted from a backlight is affected by birefringence ofthe liquid crystal molecules. In addition, since the first polarizingplate 50303 and the second polarizing plate 50304 are provided so as tobe in a cross nicol state, light emitted from the backlight passesthrough the substrate. Therefore, white display is performed. This is aso-called normally white mode.

A liquid crystal display device having a structure shown in FIG. 56A orFIG. 56B can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50301side or a second substrate 50302 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for an OCB mode.

FIGS. 56C and 56D are schematic views of cross sections of an FLC modeor an AFLC mode.

A liquid crystal layer 50310 is held between a first substrate 50311 anda second substrate 50312 which are provided so as to be opposite to eachother. A first electrode 50315 is formed on a top surface of the firstsubstrate 50311. A second electrode 50316 is formed on a top surface ofthe second substrate 50312. A first polarizing plate 50313 is providedon a surface of the first substrate 50311, which does not face theliquid crystal layer. A second polarizing plate 50314 is provided on asurface of the second substrate 50312, which does not face the liquidcrystal layer. Note that the first polarizing plate 50313 and the secondpolarizing plate 50314 are provided so as to be in a cross nicol state.

The first polarizing plate 50313 may be provided on the top surface ofthe first substrate 50311. The second polarizing plate 50314 may beprovided on the top surface of the second substrate 50312.

It is only necessary that at least one of the first electrode 50315 andthe second electrode 50316 have transparency (a transmissive orreflective liquid crystal display device). Alternatively, both the firstelectrode 50315 and the second electrode 50316 may have transparency,and part of one of the electrodes may have reflectivity (asemi-transmissive liquid crystal display device).

FIG. 56C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50315 and the second electrode50316 (referred to as a vertical electric field mode). Since liquidcrystal molecules are aligned laterally in a direction which is deviatedfrom a rubbing direction, light emitted from a backlight is affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50313 and the second polarizing plate 50314 areprovided so as to be in a cross nicol state, light emitted from thebacklight passes through the substrate. Therefore, white display isperformed.

Note that by controlling voltage applied to the first electrode 50315and the second electrode 50316, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 56D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50315 and the secondelectrode 50316. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50313 and the second polarizing plate 50314 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 56C orFIG. 56D can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50311side or a second substrate 50312 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for an FLC mode or an AFLC mode.

FIGS. 57A and 57B are schematic views of cross sections of an IPS mode.In the IPS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50400 is held between a first substrate 50401 anda second substrate 50402 which are provided so as to be opposite to eachother. A first electrode 50405 and a second electrode 50406 are formedon a top surface of the second substrate 50402. A first polarizing plate50403 is provided on a surface of the first substrate 50401, which doesnot face the liquid crystal layer. A second polarizing plate 50404 isprovided on a surface of the second substrate 50402, which does not facethe liquid crystal layer. Note that the first polarizing plate 50403 andthe second polarizing plate 50404 are provided so as to be in a crossnicol state.

The first polarizing plate 50403 may be provided on the top surface ofthe first substrate 50401. The second polarizing plate 50404 may beprovided on the top surface of the second substrate 50402.

Both the first electrode 50405 and the second electrode 50406 may havetransparency (a transmissive liquid crystal display device).Alternatively, part of one of the first electrode 50405 and the secondelectrode 50406 may have reflectivity (a semi-transmissive liquidcrystal display device).

FIG. 57A is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50405 and the second electrode50406 (referred to as a horizontal electric field mode). Since liquidcrystal molecules are aligned along a line of electric force which isdeviated from a rubbing direction, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50403 and the second polarizing plate50404 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed.

Note that by controlling voltage applied to the first electrode 50405and the second electrode 50406, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 57B is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50405 and the secondelectrode 50406. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from a backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50403 and the second polarizing plate 50404 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 57A orFIG. 57B can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50401side or a second substrate 50402 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for an IPS mode.

FIGS. 57C and 57D are schematic views of cross sections of an FFS mode.In the FFS mode, alignment of liquid crystal molecules in a liquidcrystal layer can be optically compensated, the liquid crystal moleculesare constantly rotated in a plane parallel to a substrate, and ahorizontal electric field method in which electrodes are provided onlyon one substrate side is used.

A liquid crystal layer 50410 is held between a first substrate 50411 anda second substrate 50412 which are provided so as to be opposite to eachother. A second electrode 50416 is formed on a top surface of the secondsubstrate 50412. An insulating film 50417 is formed on a top surface ofthe second electrode 50416. A first electrode 50415 is formed over theinsulating film 50417. A first polarizing plate 50413 is provided on asurface of the first substrate 50411, which does not face the liquidcrystal layer 50410. A second polarizing plate 50414 is provided on asurface of the second substrate 50412, which does not face the liquidcrystal layer 50410. Note that the first polarizing plate 50413 and thesecond polarizing plate 50414 are provided so as to be in a cross nicolstate.

The first polarizing plate 50413 may be provided on the top surface ofthe first substrate 50411, i.e., may be provided between the firstsubstrate 50411 and the liquid crystal layer. The second polarizingplate 50414 may be provided on the top surface of the second substrate50412, i.e., may be provided between the second substrate 50412 and theliquid crystal layer.

It is only necessary that at least one of the first electrode 50415 andthe second electrode 50416 have transparency (a transmissive orreflective liquid crystal display device). Alternatively, both the firstelectrode 50415 and the second electrode 50416 may have transparency,and part of one of the electrodes may have reflectivity (asemi-transmissive liquid crystal display device).

FIG. 57C is a schematic view of a cross section in the case wherevoltage is applied to the first electrode 50415 and the second electrode50416 (referred to as a horizontal electric field mode). Since liquidcrystal molecules are aligned along a line of electric force which isdeviated from a rubbing direction, light emitted from a backlight isaffected by birefringence of the liquid crystal molecules. In addition,since the first polarizing plate 50413 and the second polarizing plate50414 are provided so as to be in a cross nicol state, light emittedfrom the backlight passes through the substrate. Therefore, whitedisplay is performed.

Note that by controlling voltage applied to the first electrode 50415and the second electrode 50416, conditions of the liquid crystalmolecules can be controlled. Therefore, since the amount of lightemitted from the backlight passing through the substrate can becontrolled, predetermined image display can be performed.

FIG. 57D is a schematic view of a cross section in the case wherevoltage is not applied to the first electrode 50415 and the secondelectrode 50416. Since liquid crystal molecules are aligned laterally ina rubbing direction, light emitted from the backlight is not affected bybirefringence of the liquid crystal molecules. In addition, since thefirst polarizing plate 50413 and the second polarizing plate 50414 areprovided so as to be in a cross nicol state, light emitted from thebacklight does not pass through the substrate. Therefore, black displayis performed. This is a so-called normally black mode.

A liquid crystal display device having a structure shown in FIG. 57C orFIG. 57D can perform full-color display by being provided with a colorfilter. The color filter can be provided over a first substrate 50411side or a second substrate 50412 side.

It is only necessary that a known material be used for a liquid crystalmaterial used for an FFS mode.

Next, various liquid crystal modes are described with reference to topplan views.

FIG. 58 is a top plan view of a pixel portion to which an MVA mode isapplied. In the MVA mode, viewing angle dependency of each portion iscompensated by each other.

FIG. 58 shows a first electrode 50501, second electrodes (50502 a, 50502b, and 50502 c), and a protrusion 50503. The first electrode 50501 isformed over the entire surface of a counter substrate. The protrusion50503 is formed so as to be a dogleg shape. In addition, the secondelectrodes (50502 a, 50502 b, and 50502 c) are formed over the firstelectrode 50501 so as to have shapes corresponding to the protrusion50503.

Opening portions of the second electrodes (50502 a, 50502 b, and 50502c) function like protrusions.

In the case where voltage is applied to the first pixel electrode 50501and the second pixel electrodes (50502 a, 50502 b, and 50502 c)(referred to as a vertical electric field mode), liquid crystalmolecules are aligned so as to tilt toward the opening portions of thesecond pixel electrodes (50502 a, 50502 b, and 50502 c) and theprotrusion 50503. Since light emitted from a backlight passes through asubstrate when a pair of polarizing plates is provided so as to be in across nicol state, white display is performed.

Note that by controlling voltage applied to the first electrode 50501and the second electrodes (50502 a, 50502 b, and 50502 c), conditions ofthe liquid crystal molecules can be controlled. Therefore, since theamount of light emitted from the backlight passing through the substratecan be controlled, predetermined image display can be performed.

In the case where voltage is not applied to the first pixel electrode50501 and the second pixel electrodes (50502 a, 50502 b, and 50502 c),the liquid crystal molecules are aligned longitudinally. Since lightemitted from the backlight does not pass through a panel when the pairof polarizing plates is provided so as to be in the cross nicol state,black display is performed. This is a so-called normally black mode.

It is only necessary that a known material be used for a liquid crystalmaterial used for an MVA mode.

FIGS. 59A to 59D are top plan views of a pixel portion to which an IPSmode is applied. In the IPS mode, alignment of liquid crystal moleculesin a liquid crystal layer can be optically compensated, the liquidcrystal molecules are constantly rotated in a plane parallel to asubstrate, and a horizontal electric field method in which electrodesare provided only on one substrate side is used.

In the IPS mode, a pair of electrodes is formed so as to have differentshapes.

FIG. 59A shows a first pixel electrode 50601 and a second pixelelectrode 50602. The first pixel electrode 50601 and the second pixelelectrode 50602 are wavy shapes.

FIG. 59B shows a first pixel electrode 50611 and a second pixelelectrode 50612. The first pixel electrode 50611 and the second pixelelectrode 50612 have shapes having concentric openings.

FIG. 59C shows a first pixel electrode 50621 and a second pixelelectrode 50622. The first pixel electrode 50621 and the second pixelelectrode 50622 are comb shapes and partially overlap with each other.

FIG. 59D shows a first pixel electrode 50631 and a second pixelelectrode 50632. The first pixel electrode 50631 and the second pixelelectrode 50632 are comb shapes in which electrodes engage with eachother.

In the case where voltage is applied to the first pixel electrodes(50601, 50611, 50621, and 50631) and the second pixel electrodes (50602,50612, 50622, and 50632) (referred to as a horizontal electric fieldmode), liquid crystal molecules are aligned along a line of electricforce which is deviated from a rubbing direction. Since light emittedfrom a backlight passes through a substrate when a pair of polarizingplates is provided so as to be in a cross nicol state, white display isperformed.

Note that by controlling voltage applied to the first pixel electrodes(50601, 50611, 50621, and 50631) and the second pixel electrodes (50602,50612, 50622, and 50632), conditions of the liquid crystal molecules canbe controlled. Therefore, since the amount of light emitted from thebacklight passing through the substrate can be controlled, predeterminedimage display can be performed.

In the case where voltage is not applied to the first pixel electrodes(50601, 50611, 50621, and 50631) and the second pixel electrodes (50602,50612, 50622, and 50632), the liquid crystal molecules are alignedlaterally in the rubbing direction. Since light emitted from thebacklight does not pass through the substrate when the pair ofpolarizing plates is provided so as to be in the cross nicol state,black display is performed. This is a so-called normally black mode.

It is only necessary that a known material be used for a liquid crystalmaterial used for an IPS mode.

FIGS. 60A to 60D are top plan views of a pixel portion to which an FFSmode is applied. In the FFS mode, alignment of liquid crystal moleculesin a liquid crystal layer can be optically compensated, the liquidcrystal molecules are constantly rotated in a plane parallel to asubstrate, and a horizontal electric field method in which electrodesare provided only on one substrate side is used.

In the FFS mode, a first electrode is formed over a top surface of asecond electrode so as to be various shapes.

FIG. 60A shows a first pixel electrode 50701 and a second pixelelectrode 50702. The first pixel electrode 50701 is a bent dogleg shape.The second pixel electrode 50702 is not necessarily patterned.

FIG. 60B shows a first pixel electrode 50711 and a second pixelelectrode 50712. The first pixel electrode 50711 is a concentric shape.The second pixel electrode 50712 is not necessarily patterned.

FIG. 60C shows a first pixel electrode 50721 and a second pixelelectrode 50722. The first pixel electrode 50721 is a comb shape inwhich electrodes engage with each other. The second pixel electrode50722 is not necessarily patterned.

FIG. 60D shows a first pixel electrode 50731 and a second pixelelectrode 50732. The first pixel electrode 50731 is a comb shape. Thesecond pixel electrode 50732 is not necessarily patterned.

In the case where voltage is applied to the first pixel electrodes(50701, 50711, 50721, and 50731) and the second pixel electrodes (50702,50712, 50722, and 50732) (referred to as a horizontal electric fieldmode), liquid crystal molecules are aligned along a line of electricforce which is deviated from a rubbing direction. Since light emittedfrom a backlight passes through a substrate when a pair of polarizingplates is provided so as to be in a cross nicol state, white display isperformed.

Note that by controlling voltage applied to the first pixel electrodes(50701, 50711, 50721, and 50731) and the second pixel electrodes (50702,50712, 50722, and 50732), conditions of the liquid crystal molecules canbe controlled. Therefore, since the amount of light emitted from thebacklight passing through the substrate can be controlled, predeterminedimage display can be performed.

In the case where voltage is not applied to the first pixel electrodes(50701, 50711, 50721, and 50731) and the second pixel electrodes (50702,50712, 50722, and 50732), the liquid crystal molecules are alignedlaterally in the rubbing direction. Since light emitted from thebacklight does not pass through the substrate when the pair ofpolarizing plates is provided so as to be in the cross nicol state,black display is performed. This is a so-called normally black mode.

It is only necessary that a known material be used for a liquid crystalmaterial used for an FFS mode.

Although this embodiment mode is described with reference to variousdrawings, the contents (or may be part of the contents) described ineach drawing can be freely applied to, combined with, or replaced withthe contents (or may be part of the contents) described in anotherdrawing. Further, even more drawings can be formed by combining eachpart with another part in the above-described drawings.

The contents (or may be part of the contents) described in each drawingof this embodiment mode can be freely applied to, combined with, orreplaced with the contents (or may be part of the contents) described ina drawing in another embodiment mode. Further, even more drawings can beformed by combining each part with part of another embodiment mode inthe drawings of this embodiment mode.

This embodiment mode shows an example of an embodied case of thecontents (or may be part of the contents) described in other embodimentmodes, an example of slight transformation thereof, an example ofpartial modification thereof, an example of improvement thereof, anexample of detailed description thereof, an application example thereof,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 10

In this embodiment mode, a pixel structure of a display device isdescribed. In particular, a pixel structure of a display device using anorganic EL element is described.

FIG. 61A shows an example of a top plan view (a layout diagram) of apixel including two transistors. FIG. 61B shows an example of across-sectional view along X-X′ in FIG. 61A.

FIG. 61A shows a first transistor 60105, a first wiring 60106, a secondwiring 60107, a second transistor 60108, a third wiring 60111, a counterelectrode 60112, a capacitor 60113, a pixel electrode 60115, a partitionwall 60116, an organic conductive film 60117, an organic thin film60118, and a substrate 60119. Note that it is preferable that the firsttransistor 60105 be used as a switching transistor, the first wiring60106 as a gate signal line, the second wiring 60107 as a source signalline, the second transistor 60108 as a driving transistor, and the thirdwiring 60111 as a current supply line.

A gate electrode of the first transistor 60105 is electrically connectedto the first wiring 60106. One of a source electrode and a drainelectrode of the first transistor 60105 is electrically connected to thesecond wiring 60107. The other of the source electrode and the drainelectrode of the first transistor 60105 is electrically connected to agate electrode of the second transistor 60108 and one electrode of thecapacitor 60113. Note that the gate electrode of the first transistor60105 includes a plurality of gate electrodes. Accordingly, leakagecurrent in the off state of the first transistor 60105 can be reduced.

One of a source electrode and a drain electrode of the second transistor60108 is electrically connected to the third wiring 60111, and the otherof the source electrode and the drain electrode of the second transistor60108 is electrically connected to the pixel electrode 60115.Accordingly, current flowing to the pixel electrode 60115 can becontrolled by the second transistor 60108.

The organic conductive film 60117 is provided over the pixel electrode60115, and the organic thin film 60118 (an organic compound layer) isfurther provided thereover. The counter electrode 60112 is provided overthe organic thin film 60118 (the organic compound layer). Note that thecounter electrode 60112 may be formed over all pixels to be commonlyconnected to all the pixels, or may be patterned using a shadow mask orthe like.

Light emitted from the organic thin film 60118 (the organic compoundlayer) is transmitted through either the pixel electrode 60115 or thecounter electrode 60112.

In FIG. 61B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60115 be formed of a transparent conductive film. On the otherhand, in the case of top emission, it is preferable that the counterelectrode 60112 be formed of a transparent conductive film.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surface andlight emission of RGB can be obtained by using a color filter.

Note that the structures shown in FIGS. 61A and 61B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, as well as the structures shown in FIGS. 61A and 61B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawing.

FIG. 62A shows an example of a top plan view (a layout diagram) of apixel including three transistors. FIG. 62B shows an example of across-sectional view along X-X′ in FIG. 62A.

FIG. 62A shows a substrate 60200, a first wiring 60201, a second wiring60202, a third wiring 60203, a fourth wiring 60204, a first transistor60205, a second transistor 60206, a third transistor 60207, a pixelelectrode 60208, a partition wall 60211, an organic conductive film60212, an organic thin film 60213, and a counter electrode 60214. Notethat it is preferable that the first wiring 60201 be used as a sourcesignal line, the second wiring 60202 as a gate signal line for writing,the third wiring 60203 as a gate signal line for erasing, the fourthwiring 60204 as a current supply line, the first transistor 60205 as aswitching transistor, the second transistor 60206 as an erasingtransistor, and the third transistor 60207 as a driving transistor.

A gate electrode of the first transistor 60205 is electrically connectedto the second wiring 60202. One of a source electrode and a drainelectrode of the first transistor 60205 is electrically connected to thefirst wiring 60201. The other of the source electrode and the drainelectrode of the first transistor 60205 is electrically connected to agate electrode of the third transistor 60207. Note that the gateelectrode of the first transistor 60205 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the firsttransistor 60205 can be reduced.

A gate electrode of the second transistor 60206 is electricallyconnected to the third wiring 60203. One of a source electrode and adrain electrode of the second transistor 60206 is electrically connectedto the fourth wiring 60204. The other of the source electrode and thedrain electrode of the second transistor 60206 is electrically connectedto the gate electrode of the third transistor 60207. Note that the gateelectrode of the second transistor 60206 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the secondtransistor 60206 can be reduced.

One of a source electrode and a drain electrode of the third transistor60207 is electrically connected to the fourth wiring 60204, and theother of the source electrode and the drain electrode of the thirdtransistor 60207 is electrically connected to the pixel electrode 60208.Accordingly, current flowing to the pixel electrode 60208 can becontrolled by the third transistor 60207.

The organic conductive film 60212 is provided over the pixel electrode60208, and the organic thin film 60213 (an organic compound layer) isfurther provided thereover. The counter electrode 60214 is provided overthe organic thin film 60213 (the organic compound layer). Note that thecounter electrode 60214 may be formed over all pixels to be commonlyconnected to all the pixels, or may be patterned using a shadow mask orthe like.

Light emitted from the organic thin film 60213 (the organic compoundlayer) is transmitted through either the pixel electrode 60208 or thecounter electrode 60214.

In FIG. 62B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60208 be formed of a transparent conductive film. On the otherhand, in the case of top emission, it is preferable that the counterelectrode 60214 be formed of a light-transmitting conductive film.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surface andlight emission of RGB can be obtained by using a color filter.

Note that the structures shown in FIGS. 62A and 62B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, as well as the structures shown in FIGS. 62A and 62B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawings.

FIG. 63A shows an example of a top plan view (a layout diagram) of apixel including four transistors. FIG. 63B shows an example of across-sectional view along X-X′ in FIG. 63A.

FIG. 63A shows a substrate 60300, a first wiring 60301, a second wiring60302, a third wiring 60303, a fourth wiring 60304, a first transistor60305, a second transistor 60306, a third transistor 60307, a fourthtransistor 60308, a pixel electrode 60309, a fifth wiring 60311, a sixthwiring 60312, a partition wall 60321, an organic conductive film 60322,an organic thin film 60323, and a counter electrode 60324. Note that itis preferable that the first wiring 60301 be used as a source signalline, the second wiring 60302 as a gate signal line for writing, thethird wiring 60303 as a gate signal line for erasing, the fourth wiring60304 as a signal line for reverse bias, the first transistor 60305 as aswitching transistor, the second transistor 60306 as an erasingtransistor, the third transistor 60307 as a driving transistor, thefourth transistor 60308 as a transistor for reverse bias, the fifthwiring 60311 as a current supply line, and the sixth wiring 60312 as apower supply line for reverse bias.

A gate electrode of the first transistor 60305 is electrically connectedto the second wiring 60302. One of a source electrode and a drainelectrode of the first transistor 60305 is electrically connected to thefirst wiring 60301. The other of the source electrode and the drainelectrode of the first transistor 60305 is electrically connected to agate electrode of the third transistor 60307. Note that the gateelectrode of the first transistor 60305 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the firsttransistor 60305 can be reduced.

A gate electrode of the second transistor 60306 is electricallyconnected to the third wiring 60303. One of a source electrode and adrain electrode of the second transistor 60306 is electrically connectedto the fifth wiring 60311. The other of the source electrode and thedrain electrode of the second transistor 60306 is electrically connectedto the gate electrode of the third transistor 60307. Note that the gateelectrode of the second transistor 60306 includes a plurality of gateelectrodes. Accordingly, leakage current in the off state of the secondtransistor 60306 can be reduced.

One of a source electrode and a drain electrode of the third transistor60307 is electrically connected to the fifth wiring 60311, and the otherof the source electrode and the drain electrode of the third transistor60307 is electrically connected to the pixel electrode 60309.Accordingly, current flowing to the pixel electrode 60309 can becontrolled by the third transistor 60307.

A gate electrode of the fourth transistor 60308 is electricallyconnected to the fourth wiring 60304. One of a source electrode and adrain electrode of the fourth transistor 60308 is electrically connectedto the sixth wiring 60312. The other of the source electrode and thedrain electrode of the fourth transistor 60308 is electrically connectedto the pixel electrode 60309. Accordingly, a potential of the pixelelectrode 60309 can be controlled by the fourth transistor 60308, sothat a reverse bias can be applied to the organic conductive film 60322and the organic thin film 60323. When a reverse bias is applied to alight-emitting element including the organic conductive film 60322, theorganic thin film 60323, and the like, reliability of the light-emittingelement can be significantly improved.

The organic conductive film 60322 is provided over the pixel electrode60309, and the organic thin film 60323 (an organic compound layer) isfurther provided thereover. The counter electrode 60324 is provided overthe organic thin film 60213 (the organic compound layer). Note that thecounter electrode 60324 may be formed over all pixels to be commonlyconnected to all the pixels, or may be patterned using a shadow mask orthe like.

Light emitted from the organic thin film 60323 (the organic compoundlayer) is transmitted through either the pixel electrode 60309 or thecounter electrode 60324.

In FIG. 63B, the case where light is emitted to the pixel electrodeside, that is, a side on which the transistor and the like are formed isreferred to as bottom emission; and the case where light is emitted tothe counter electrode side is referred to as top emission.

In the case of bottom emission, it is preferable that the pixelelectrode 60309 be formed of a transparent conductive film. On the otherhand, in the case of top emission, it is preferable that the counterelectrode 60324 be formed of a light-transmitting conductive film.

In a light-emitting device for color display, EL elements havingrespective light emission colors of RGB may be separately formed, or anEL element with a single color may be formed over an entire surface andlight emission of RGB can be obtained by using a color filter.

Note that the structures shown in FIGS. 63A and 63B are examples, andvarious structures can be employed for a pixel layout, a cross-sectionalstructure, a stacking order of electrodes of an EL element, and thelike, as well as the structures shown in FIGS. 63A and 63B. Further, asa light-emitting element, various elements such as a crystalline elementsuch as an LED, and an element formed of an inorganic thin film can beused as well as the element formed of the organic thin film shown in thedrawings.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to; combinedwith, or replaced with this embodiment mode.

Embodiment Mode 11

In this embodiment mode, a structure and an operation of a pixel in adisplay device are described.

FIGS. 64A and 64B are timing charts showing an example of digital timegray scale drive. The timing chart of FIG. 64A shows a driving methodwhen a signal writing period (an address period) to a pixel and alight-emitting period (a sustain period) are separated.

One frame period refers to a period for fully displaying an image forone display region. One frame period includes a plurality of subframeperiods, and one subframe period includes an address period and asustain period. Address periods Ta1 to Ta4 indicate time for writingsignals to pixels in all rows, and periods Tb1 to Tb4 indicate time forwriting signals to pixels in one row (or one pixel). Sustain periods Ts1to Ts4 indicate time for maintaining a lighting state or a non-lightingstate in accordance with a video signal written to the pixel, and aratio of the length of the Sustain periods is set to satisfyTs1:Ts2:Ts3:Ts4=2³:2²:2¹:2⁰=8:4:2:1. A gray scale is expressed dependingon in which sustain period light emission is performed.

Here, the i-th pixel row is described with reference to FIG. 64B. First,in the address period Ta1, a pixel selection signal is input to a scanline in order from a first row, and in a period Tb1(i) in the addressperiod Ta1, a pixel in the i-th row is selected. Then, while the pixelin the i-th row is selected, a video signal is input to the pixel in thei-th row from a signal line. Then, when the video signal is written tothe pixel in the i-th row, the pixel in the i-th row maintains thesignal until a signal is input again. Lighting and non-lighting of thepixel in the i-th row in the sustain period Ts1 are controlled by thewritten video signal. Similarly, in the address periods Ta2, Ta3, andTa4, a video signal is input to the pixel in the i-th row, and lightingand non-lighting of the pixel in the i-th row in the sustain periodsTs2, Ts3, and Ts4 are controlled by the video signal. Then, in eachsubframe period, a pixel to which a signal for not lighting in theaddress period and for lighting when the sustain period starts after theaddress period ends is written is lit.

Here, the case where a 4-bit gray scale is expressed is described;however, the number of bits and the number of gray scales are notlimited thereto. Note that lighting is not needed to be performed inorder of Ts1, Ts2, Ts3, and Ts4, and the order may be random or lightemission may be performed in the period divided into a plurality ofperiods. A ratio of lighting time of Ts1, Ts2, Ts3, and Ts4 is notneeded to be power-of-two, and may be the same length or slightlydifferent from a power of two.

Next, a driving method when a signal writing period (an address period)to a pixel and a light-emitting period (a sustain period) are notseparated is described. A pixel in a row in which a writing operation ofa video signal is completed maintains the signal until another signal iswritten to the pixel (or the signal is erased). Data holding time refersto a period from the writing operation is performed until another signalis written to the pixel. In the data holding time, the pixel is lit ornot lit in accordance with the video signal written to the pixel. Thesame operations are performed until the last row, and the address periodends. Then, an operation proceeds to a signal writing operation in anext subframe period sequentially from a row in which the data holdingtime ends.

As described above, in the case of a driving method in which a pixel isimmediately lit or not lit in accordance with a video signal written tothe pixel after the signal writing operation is completed and the dataholding time starts, signals cannot be input to two rows at the sametime. Accordingly, address periods need to be prevented fromoverlapping, so that the data holding time cannot be made shorter thanthe address period. As a result, it becomes difficult to performhigh-level gray scale display.

Thus, the data holding time is set to be shorter than the address periodby provision of an erasing period. A driving method when the dataholding time is set shorter than the address period by provision of anerasing period is described with reference to FIG. 65A.

First, in the address period Ta1, a pixel scan signal is input to a scanline in order from a first row, and a pixel is selected. Then, while thepixel is selected, a video signal is input to the pixel from a signalline. Then, when the video signal is written to the pixel, the pixelmaintains the signal until a signal is input again. Lighting andnon-lighting of the pixel in the sustain period Ts1 are controlled bythe written video signal. In a row in which a writing operation of avideo signal is completed, a pixel is immediately lit or not lit inaccordance with the written video signal. The same operations areperformed until the last row, and the address period Ta1 ends. Then, anoperation proceeds to a signal writing operation in a next subframeperiod sequentially from a row in which the data holding time ends.Similarly, in the address periods Ta2, Ta3, and Ta4, a video signal isinput to the pixel, and lighting and non-lighting of the pixel in thesustain periods Ts2, Ts3, and Ts4 are controlled by the video signal.The end of the sustain period Ts4 is set by the start of an erasingoperation. This is because when a signal written to a pixel in anerasing time Te of each row is erased, the pixel is forced to be not litregardless of the video signal written to the pixel in the addressperiod until another signal is written to the pixel. That is, the dataholding time ends from a pixel in which the erasing time Te starts.

Here, the i-th pixel row is described with reference to FIG. 65B. In theaddress period Ta1, a pixel scan signal is input to a scan line in orderfrom a first row, and a pixel is selected. Then, in the period Tb1(i),while the pixel in the i-th row is selected, a video signal is input tothe pixel in the i-th row. Then, when the video signal is written to thepixel in the i-th row, the pixel in the i-th row maintains the signaluntil a signal is input again. Lighting and non-lighting of the pixel inthe i-th row in a sustain period Ts1(i) are controlled by the writtenvideo signal. That is, the pixel in the i-th row is immediately lit ornot lit in accordance with the video signal written to the pixel afterthe writing operation of the video signal to the i-th row is completed.Similarly, in the address periods Ta2, Ta3, and Ta4, a video signal isinput to the pixel in the i-th row, and lighting and non-lighting of thepixel in the i-th row in the sustain periods Ts2, Ts3, and Ts4 arecontrolled by the video signal. The end of a sustain period Ts4(i) isset by the start of an erasing operation. This is because the pixel isforced to be not lit regardless of the video signal written to the pixelin the i-th row in an erasing time Te(i) in the i-th row. That is, thedata holding time of the pixel in the i-th row ends when the erasingtime Te(i) starts.

Thus, a display device with a high-level gray scale and a high dutyratio (a ratio of a lighting period in one frame period) can beprovided, in which data holding time is shorter than an address periodwithout separating the address period and a sustain period. Sinceinstantaneous luminance can be lowered, reliability of a display elementcan be improved.

Here, the case where a 4-bit gray scale is expressed is described;however, the number of bits and the number of gray scales are notlimited thereto. Note that lighting is not needed to be performed inorder of Ts1, Ts2, Ts3, and Ts4, and the order may be random or lightemission may be performed in the period divided into a plurality ofperiods. A ratio of lighting time of Ts1, Ts2, Ts3, and Ts4 is notneeded to be power-of-two, and may be the same length or slightlydifferent from a power of two.

Next, a structure and an operation of a pixel to which digital time grayscale drive can be applied are described.

FIG. 66 is a diagram showing an example of a pixel structure to whichdigital time gray scale drive can be applied.

A pixel 80300 includes a switching transistor 80301, a drivingtransistor 80302, a light-emitting element 80304, and a capacitor 80303.A gate of the switching transistor 80301 is connected to a scan line80306, a first electrode (one of a source electrode and a drainelectrode) of the switching transistor 80301 is connected to a signalline 80305, and a second electrode (the other of the source electrodeand the drain electrode) of the switching transistor 80301 is connectedto a gate of the driving transistor 80302. The gate of the drivingtransistor 80302 is connected to a power supply line 80307 through thecapacitor 80303, a first electrode of the driving transistor 80302 isconnected to the power supply line 80307, and a second electrode of thedriving transistor 80302 is connected to a first electrode (a pixelelectrode) of the light-emitting element 80304. A second electrode ofthe light-emitting element 80304 corresponds to a common electrode80308.

Note that the second electrode (the common electrode 80308) of thelight-emitting element 80304 is set to a low power supply potential. Thelow power supply potential refers to a potential satisfying (the lowpower supply potential)<(a high power supply potential) based on thehigh power supply potential set to the power supply line 80307. As thelow power supply potential, GND, 0 V, or the like may be set, forexample. A potential difference between the high power supply potentialand the low power supply potential is applied to the light-emittingelement 80304, and current is supplied to the light-emitting element80304. Here, in order to make the light-emitting element 80304 emitlight, each potential is set so that the potential difference betweenthe high power supply potential and the low power supply potential isforward threshold voltage or more.

Note that gate capacitance of the driving transistor 80302 may be usedas a substitute for the capacitor 80303, so that the capacitor 80303 canbe omitted. The gate capacitance of the driving transistor 80302 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel region and the gate electrode.

When a pixel is selected by the scan line 80306, that is, when theswitching transistor 80301 is turned on, a video signal is input to thepixel from the signal line 80305. Then, a charge for voltagecorresponding to the video signal is stored in the capacitor 80303, andthe capacitor 80303 maintains the voltage. The voltage is voltagebetween the gate electrode and the first electrode of the drivingtransistor 80302 and corresponds to gate-source voltage Vgs of thedriving transistor 80302.

An operation region of a transistor can be generally divided into alinear region and a saturation region. When drain-source voltage isdenoted by Vds, gate-source voltage is denoted by Vgs, and thresholdvoltage is denoted by Vth, a boundary between the linear region and thesaturation region sets so as to satisfy (Vgs−Vth)=Vds. In the case where(Vgs−Vth)>Vds is satisfied, a transistor operates in a linear region,and a current value is determined in accordance with the level of Vdsand Vgs. On the other hand, in the case where (Vgs−Vth)<Vds issatisfied, a transistor operates in a saturation region and ideally, acurrent value hardly changes even when Vds changes. That is, a currentvalue is determined only by the level of Vgs.

Here, in the case of voltage-input voltage driving method, a videosignal is input to the gate of the driving transistor 80302 so that thedriving transistor 80302 is in either of two states of beingsufficiently turned on and turned off. That is, the driving transistor80302 operates in a linear region.

Thus, when a video signal which makes the driving transistor 80302turned on is input, a power supply potential VDD set to the power supplyline 80307 without change is ideally set to the first electrode of thelight-emitting element 80304.

That is, ideally, constant voltage is applied to the light-emittingelement 80304 to obtain constant luminance from the light-emittingelement 80304. Then, a plurality of subframe periods are provided in oneframe period. A video signal is written to a pixel in each subframeperiod, lighting and non-lighting of the pixel are controlled in eachsubframe period, and a gray scale is expressed by the sum of lightingsubframe periods.

Note that when the video signal by which the driving transistor 80302operates in a saturation region is input, current can be supplied to thelight-emitting element 80304. When the light-emitting element 80304 isan element luminance of which is determined in accordance with current,luminance decay due to deterioration of the light-emitting element 80304can be suppressed. Further, when the video signal is an analog signal,current in accordance with the video signal can be supplied to thelight-emitting element 80304. In this case, analog gray scale drive canbe performed.

FIG. 67 is a diagram showing another example of a pixel structure towhich digital time gray scale drive can be applied.

A pixel 80400 includes a switching transistor 80401, a drivingtransistor 80402, a capacitor 80403, a light-emitting element 80404, anda rectifying element 80409. A gate of the switching transistor 80401 isconnected to a first scan line 80406, a first electrode (one of a sourceelectrode and a drain electrode) of the switching transistor 80401 isconnected to a signal line 80405, and a second electrode (the other ofthe source electrode and the drain electrode) of the switchingtransistor 80401 is connected to a gate of the driving transistor 80402.The gate of the driving transistor 80402 is connected to a power supplyline 80407 through the capacitor 80403, and is also connected to asecond scan line 80410 through the rectifying element 80409. A firstelectrode of the driving transistor 80402 is connected to the powersupply line 80407, and a second electrode of the driving transistor80402 is connected to a first electrode (a pixel electrode) of thelight-emitting element 80404. A second electrode of the light-emittingelement 80404 corresponds to a common electrode 80408.

The second electrode (the common electrode 80408) of the light-emittingelement 80404 is set to a low power supply potential. Note that the lowpower supply potential refers to a potential satisfying (the low powersupply potential)<(a high power supply potential) with respect to thehigh power supply potential set to the power supply line 80407. As thelow power supply potential, GND, 0 V, or the like may be set, forexample. In order to apply a potential difference between the high powersupply potential and the low power supply potential to thelight-emitting element 80404 and supply current to the light-emittingelement 80404 so that the light-emitting element 80404 emits light, eachpotential is set so that the potential difference between the high powersupply potential and the low power supply potential is equal to forwardthreshold voltage or more.

Gate capacitance of the driving transistor 80402 may be used as asubstitute for the capacitor 80403, so that the capacitor 80403 can beomitted. The gate capacitance of the driving transistor 80402 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel region and the gate electrode.

As the rectifying element 80409, a diode-connected transistor can beused. A PN junction diode, a PIN junction diode, a Schottky diode, adiode formed of a carbon nanotube, or the like may be used other than adiode-connected transistor. A diode-connected transistor may be ann-channel transistor or a p-channel transistor.

The pixel 80400 is such that the rectifying element 80409 and the secondscan line 80410 are added to the pixel shown in FIG. 66. Accordingly,the switching transistor 80401, the driving transistor 80402, thecapacitor 80403, the light-emitting element 80404, the signal line80405, the first scan line 80406, the power supply line 80407, and thecommon electrode 80408 shown in FIG. 67 correspond to the switchingtransistor 80301, the driving transistor 80302, the capacitor 80303, thelight-emitting element 80304, the signal line 80305, the scan line80306, the power supply line 80307, and the common electrode 80308 shownin FIG. 66. Accordingly, a writing operation and a light-emittingoperation in FIG. 67 are similar to those described in FIG. 66, so thatdescription thereof is omitted.

An erasing operation of the pixel shown in FIG. 67 is described. In anerasing operation, an H-level signal is input to the second scan line80410. Thus, current is supplied to the rectifying element 80409, and agate potential of the driving transistor 80402 held by the capacitor80403 can be set to a certain potential. That is, the potential of thegate electrode of the driving transistor 80402 is set to a certainvalue, and the driving transistor 80402 can be forced to be turned offregardless of a video signal written to the pixel.

An L-level signal input to the second scan line 80410 has a potentialsuch that current is not supplied to the rectifying element 80409 when avideo signal for non-lighting is written to a pixel. An H-level signalinput to the second scan line 80410 has a potential such that apotential to turn off the driving transistor 80302 can be set to thegate regardless of a video signal written to a pixel.

FIG. 68 is a diagram showing another example of a pixel structure towhich digital time gray scale drive can be applied.

A pixel 80500 includes a switching transistor 80501, a drivingtransistor 80502, a capacitor 80503, a light-emitting element 80504, andan erasing transistor 80509. A gate of the switching transistor 80501 isconnected to a first scan line 80506, a first electrode (one of a sourceelectrode and a drain electrode) of the switching transistor 80501 isconnected to a signal line 80505, and a second electrode (the other ofthe source electrode and the drain electrode) of the switchingtransistor 80501 is connected to a gate of the driving transistor 80502.The gate of the driving transistor 80502 is connected to a power supplyline 80507 through the capacitor 80503, and is also connected to a firstelectrode of the erasing transistor 80509. A first electrode of thedriving transistor 80502 is connected to the power supply line 80507,and a second electrode of the driving transistor 80502 is connected to afirst electrode (a pixel electrode) of the light-emitting element 80504.A gate of the erasing transistor 80509 is connected to a second scanline 80510, and a second electrode of the erasing transistor 80509 isconnected to the power supply line 80507. A second electrode of thelight-emitting element 80504 corresponds to a common electrode 80508.

The second electrode (the common electrode 80508) of the light-emittingelement 80504 is set to a low power supply potential. The low powersupply potential refers to a potential satisfying (the low power supplypotential)<(a high power supply potential) with respect to the highpower supply potential set to the power supply line 80507. As the lowpower supply potential, GND, 0 V, or the like may be set, for example.In order to apply a potential difference between the high power supplypotential and the low power supply potential to the light-emittingelement 80504 and supply current to the light-emitting element 80504 sothat the light-emitting element 80504 emits light, each potential is setso that the potential difference between the high power supply potentialand the low power supply potential is equal to forward threshold voltageor more.

Gate capacitance of the driving transistor 80502 may be used as asubstitute for the capacitor 80503, so that the capacitor 80503 can beomitted. The gate capacitance of the driving transistor 80502 may beformed in a region where a source region, a drain region, an LDD region,or the like overlaps with the gate electrode. Alternatively, capacitancemay be formed between a channel region and the gate electrode.

As the erasing transistor 80509, a diode-connected transistor can beused. Further, a PN junction diode, a PIN junction diode, a Schottkydiode, a diode formed of a carbon nanotube, or the like may be usedother than a diode-connected transistor. A diode-connected transistormay be an n-channel transistor or a p-channel transistor.

The pixel 80500 is such that the erasing transistor 80509 and the secondscan line 80510 are added to the pixel shown in FIG. 66. Accordingly,the switching transistor 80501, the driving transistor 80502, thecapacitor 80503, the light-emitting element 80504, the signal line80505, the first scan line 80506, the power supply line 80507, and thecommon electrode 80508 shown in FIG. 68 correspond to the switchingtransistor 80301, the driving transistor 80302, the capacitor 80303, thelight-emitting element 80304, the signal line 80305, the scan line80306, the power supply line 80307, and the common electrode 80308 shownin FIG. 66. Accordingly, a writing operation and a light-emittingoperation in FIG. 68 are similar to those described in FIG. 66, so thatdescription thereof is omitted.

An erasing operation of the pixel shown in FIG. 68 is described. In anerasing operation, an H-level signal is input to the second scan line80510. Thus, the erasing transistor 80509 is turned on, and the gateelectrode and the first electrode of the driving transistor can be madeto have the same potential. That is, Vgs of the driving transistor 80502can be 0 V. Accordingly, the driving transistor 80502 can be forced tobe turned off.

Next, a structure and an operation of a pixel called a threshold voltagecompensation pixel are described. A threshold voltage compensation pixelcan be applied to digital time gray scale drive and analog gray scaledrive.

FIG. 69 is a diagram showing an example of a structure of a pixel calleda threshold voltage compensation pixel.

The pixel in FIG. 69 includes a driving transistor 80600, a first switch80601, a second switch 80602, a third switch 80603, a first capacitor80604, a second capacitor 80605, and a light-emitting element 80620. Agate electrode of the driving transistor 80600 is connected to a signalline 80611 through the first capacitor 80604 and the first switch 80601in this order. Further, the gate electrode of the driving transistor80600 is connected to a power supply line 80612 through the secondcapacitor 80605. A first electrode of the driving transistor 80600 isconnected to the power supply line 80612. A second electrode of thedriving transistor 80600 is connected to a first electrode of thelight-emitting element 80620 through the third switch 80603. Further,the second electrode of the driving transistor 80600 is connected to thegate electrode of the driving transistor 80600 through the second switch80602. A second electrode of the light-emitting element 80620corresponds to a common electrode 80621.

The second electrode of the light-emitting element 80620 is set to a lowpower supply potential. Note that the low power supply potential refersto a potential satisfying (the low power supply potential)<(a high powersupply potential) based on the high power supply potential set to thepower supply line 80612. As the low power supply potential, GND, 0 V, orthe like may be set, for example. In order to apply a potentialdifference between the high power supply potential and the low powersupply potential to the light-emitting element 80620 and supply currentto the light-emitting element 80620 so that the light-emitting element80620 emits light, each potential is set so that the potentialdifference between the high power supply potential and the low powersupply potential is equal to forward threshold voltage or more. Notethat gate capacitance of the driving transistor 80600 may be used as asubstitute for the second capacitor 80605, so that the second capacitor80605 can be omitted. The gate capacitance of the driving transistor80600 may be formed in a region where a source region, a drain region,an LDD region, or the like overlaps with the gate electrode.Alternatively, capacitance may be formed between a channel region andthe gate electrode. Note that on/off of the first switch 80601, thesecond switch 80602, and the third switch 80603 is controlled by a firstscan line 80613, a second scan line 80615, and a third scan line 80614,respectively.

A method for driving the pixel shown in FIG. 69 is described in which anoperation period is divided into an initialization period, a datawriting period, a threshold detecting period, and a light-emittingperiod.

In the initialization period, the second switch 80602 and the thirdswitch 80603 are turned on. Then, a potential of the gate electrode ofthe driving transistor 80600 is lower than at least a potential of thepower supply line 80612. At this time, the first switch 80601 may be inan on state or an off state. Note that the initialization period is notnecessarily required.

In the threshold detecting period, a pixel is selected by the first scanline 80613. That is, the first switch 80601 is turned on, and a certainconstant voltage is input from the signal line 80611. At this time, thesecond switch 80602 is turned on and the third switch 80603 is turnedoff. Accordingly, the driving transistor 80600 is diode-connected, andthe second electrode and the gate electrode of the driving transistor80600 are placed in a floating state. Then, a potential of the gateelectrode of the driving transistor 80600 is a value obtained bysubtracting threshold voltage of the driving transistor 80600 from thepotential of the power supply line 80612. Thus, the threshold voltage ofthe driving transistor 80600 is held in the first capacitor 80604. Apotential difference between the potential of the gate electrode of thedriving transistor 80600 and the constant voltage input from the signalline 80611 is held in the second capacitor 80605.

In the data writing period, a video signal (voltage) is input from thesignal line 80611. At this time, the first switch 80601 is kept on, thesecond switch 80602 is turned off, and the third switch 80603 is keptoff. Since the gate electrode of the driving transistor 80600 is in afloating state, the potential of the gate electrode of the drivingtransistor 80600 changes depending on a potential difference between theconstant voltage input from the signal line 80611 in the thresholddetecting period and a video signal input from the signal line 80611 inthe data writing period. For example, when (a capacitance value of thefirst capacitor 80604)<<(a capacitance value of the second capacitor80605) is satisfied, the potential of the gate electrode of the drivingtransistor 80600 in the data writing period is approximately equal tothe sum of a potential difference (the amount of change) between thepotential of the signal line 80611 in the threshold detecting period andthe potential of the signal line 80611 in the data writing period; and avalue obtained by subtracting the threshold voltage of the drivingtransistor 80600 from the potential of the power supply line 80612. Thatis, the potential of the gate electrode of the driving transistor 80600becomes a potential obtained by correcting the threshold voltage of thedriving transistor 80600.

In the light-emitting period, current in accordance with a potentialdifference (Vgs) between the gate electrode of the driving transistor80600 and the power supply line 80612 is supplied to the light-emittingelement 80620. At this time, the first switch 80601 is turned off, thesecond switch 80602 is kept off, and the third switch 80603 is turnedon. Note that current flowing to the light-emitting element 80620 isconstant regardless of the threshold voltage of the driving transistor80600.

Note that a pixel structure of the invention is not limited to thatshown in FIG. 69. For example, a switch, a resistor, a capacitor, atransistor, a logic circuit, or the like may be added to the pixel inFIG. 69. For example, the second switch 80602 may include a p-channeltransistor or an n-channel transistor, the third switch 80603 mayinclude a transistor with polarity different from that of the secondswitch 80602, and the second switch 80602 and the third switch 80603 maybe controlled by the same scan line.

A structure and an operation of a pixel called a current input pixel aredescribed. A current input pixel can be applied to digital gray scaledrive and analog gray scale drive.

FIG. 70 is a diagram showing an example of a structure of a currentinput pixel.

The pixel shown in FIG. 70 includes a driving transistor 80700, a firstswitch 80701, a second switch 80702, a third switch 80703, a capacitor80704, and a light-emitting element 80730. A gate electrode of thedriving transistor 80700 is connected to a signal line 80711 through thesecond switch 80702 and the first switch 80701 in this order. Further,the gate electrode of the driving transistor 80700 is connected to apower supply line 80712 through the capacitor 80704. A first electrodeof the driving transistor 80700 is connected to the power supply line80712. A second electrode of the driving transistor 80700 is connectedto the signal line 80711 through the first switch 80701. Further, thesecond electrode of the driving transistor 80700 is connected to a firstelectrode of the light-emitting element 80730 through the third switch80703. A second electrode of the light-emitting element 80730corresponds to a common electrode 80731.

The second electrode of the light-emitting element 80730 is set to a lowpower supply potential. Note that the low power supply potential refersto a potential satisfying (the low power supply potential)<(a high powersupply potential) based on the high power supply potential set to thepower supply line 80712. As the low power supply potential, GND, 0 V, orthe like may be set, for example. In order to apply potential differencebetween the high power supply potential and the low power supplypotential to the light-emitting element 80730 and supply current to thelight-emitting element 80730 so that the light-emitting element 80730emits light, each potential is set so that the potential differencebetween the high power supply potential and the low power supplypotential is equal to forward threshold voltage or more. Note that gatecapacitance of the driving transistor 80700 may be used as a substitutefor the capacitor 80704, so that the capacitor 80704 can be omitted. Thegate capacitance of the driving transistor 80700 may be formed in aregion where a source region, a drain region, an LDD region, or the likeoverlaps with the gate electrode. Alternatively, capacitance may beformed between a channel region and the gate electrode. Note that on/offof the first switch 80701, the second switch 80702, and the third switch80703 is controlled by a first scan line 80713, a second scan line80714, and a third scan line 80715, respectively.

A method for driving the pixel shown in FIG. 70 is described in which anoperation period is divided into a data writing period and alight-emitting period.

In the data writing period, a pixel is selected by the first scan line80713. That is, the first switch 80701 is turned on, and current isinput as a video signal from the signal line 80711. At this time, thesecond switch 80702 is turned on and the third switch 80703 is turnedoff. Accordingly, a potential of the gate electrode of the drivingtransistor 80700 becomes a potential in accordance with the videosignal. That is, voltage between the gate electrode and the sourceelectrode of the driving transistor 80700 which is such that the drivingtransistor 80700 supplies current same as the video signal is held inthe capacitor 80704.

Next, in the light-emitting period, the first switch 80701 and thesecond switch 80702 are turned off, and the third switch 80703 is turnedon. Thus, current with the same value as the video signal is supplied tothe light-emitting element 80730.

Note that the invention is not limited to the pixel structure shown inFIG. 70. For example, a switch, a resistor, a capacitor, a transistor, alogic circuit, or the like may be added to the pixel in FIG. 70. Forexample, the first switch 80701 may include a p-channel transistor or ann-channel transistor, the second switch 80702 may include a transistorwith the same polarity as that of the first switch 80701, and the firstswitch 80701 and the second switch 80702 may be controlled by the samescan line. The second switch 80702 may be provided between the gateelectrode of the driving transistor 80700 and the signal line 80711.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 12

In this embodiment mode, a structure and a manufacturing method of atransistor are described.

FIGS. 71A to 71G show examples of structures and manufacturing methodsof transistors included in a semiconductor device to which the inventioncan be applied. FIG. 71A shows structure examples of transistorsincluded in a semiconductor device to which the invention can beapplied. FIGS. 71B to 71G show an example of a manufacturing method ofthe transistors included in a semiconductor device to which theinvention can be applied.

Note that the structure and the manufacturing method of the transistorsincluded in a semiconductor device to which the invention can be appliedare not limited to those shown in FIGS. 71A to 71G, and variousstructures and manufacturing methods can be employed.

First, structure examples of transistors included in a semiconductordevice to which the invention can be applied are described withreference to FIG. 71A. FIG. 71A is a cross-sectional view of a pluralityof transistors each having a different structure. Here, in FIG. 71A, theplurality of transistors each having a different structure arejuxtaposed, which is for describing structures of the transistors.Accordingly, the transistors are not needed to be actually juxtaposed asshown in FIG. 71A and can be separately formed as needed.

Next, characteristics of each layer forming the transistor included in asemiconductor device to which the invention can be applied aredescribed.

A substrate 110111 can be a glass substrate using barium borosilicateglass, aluminoborosilicate glass, or the like, a quartz substrate, aceramic substrate, a metal substrate containing stainless steel, or thelike. In addition, a substrate formed of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN), orpolyethersulfone (PES), or a substrate formed of a flexible syntheticresin such as acrylic can also be used. By using a flexible substrate, asemiconductor device capable of being bent can be formed. A flexiblesubstrate has no strict limitations on the area or a shape of thesubstrate. Accordingly, for example, when a substrate having arectangular shape, each side of which is 1 meter or more, is used as thesubstrate 110111, productivity can be significantly improved. Such anadvantage is highly favorable as compared with the case where a circularsilicon substrate is used.

An insulating film 110112 functions as a base film and is provided toprevent alkali metal such as Na or alkaline earth metal from thesubstrate 110111 from adversely affecting characteristics of asemiconductor element. The insulating film 110112 can have asingle-layer structure or a stacked-layer structure of an insulatingfilm containing oxygen or nitrogen, such as silicon oxide (SiOx),silicon nitride (SiNx), silicon oxynitride (SiOxNy) (x>y), or siliconnitride oxide (SiNxOy) (x>y). For example, when the insulating film110112 is provided to have a two-layer structure, it is preferable thata silicon nitride oxide film be used as a first insulating film and asilicon oxynitride film be used as a second insulating film. Further,when the insulating film 110112 is provided to have a three-layerstructure, it is preferable that a silicon oxynitride film be used as afirst insulating film, a silicon nitride oxide film be used as a secondinsulating film, and a silicon oxynitride film be used as a thirdinsulating film.

Semiconductor layers 110113, 110114, and 110115 can be formed using anamorphous semiconductor or a semi-amorphous semiconductor (SAS).Alternatively, a polycrystalline semiconductor layer may be used. SAS isa semiconductor having an intermediate structure between amorphous andcrystalline (including single crystal and polycrystalline) structuresand having a third state which is stable in free energy. Moreover, SASincludes a crystalline region with a short-range order and latticedistortion. A crystalline region of 0.5 to 20 nm can be observed atleast in part of a film. When silicon is contained as a main component,Raman spectrum shifts to a wave number side lower than 520 cm⁻¹. Thediffraction peaks of (111) and (220) which are thought to be contributedto a silicon crystalline lattice are observed by X-ray diffraction. SAScontains hydrogen or halogen of at least 1 atomic % or more tocompensate dangling bonds. SAS is formed by glow discharge decomposition(plasma CVD) of a material gas. As the material gas, Si₂H₆, SiH₂Cl₂,SiHCl₃, SiCl₄, SiF₄, or the like as well as SiH₄ can be used.Alternatively, GeF₄ may be mixed. The material gas may be diluted withH₂, or H₂ and one or more kinds of rare gas elements selected from He,Ar, Kr, and Ne. A dilution ratio is in the range of 2 to 1000 times.Pressure is in the range of approximately 0.1 to 133 Pa, and a powersupply frequency is 1 to 120 MHz, preferably 13 to 60 MHz. A substrateheating temperature may be 300° C. or lower. A concentration ofimpurities in atmospheric components such as oxygen, nitrogen, andcarbon is preferably 1×10²⁰ cm⁻¹ or less as impurity elements in thefilm. In particular, an oxygen concentration is 5×10¹⁹/cm³ or less,preferably 1×10¹⁹/cm³ or less. Here, an amorphous semiconductor layer isformed using a material containing silicon (Si) as its main component(e.g., Si_(x)Ge_(1-x)) by a known method (such as a sputtering method,an LPCVD method, or a plasma CVD method). Then, the amorphoussemiconductor layer is crystallized by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, or a thermal crystallization methodusing a metal element which promotes crystallization.

An insulating film 110116 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxynitride (SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y).

A gate electrode 110117 can have a single-layer structure of aconductive film or a stacked-layer structure of two or three conductivefilms. As a material for the gate electrode 110117, a known conductivefilm can be used. For example, a single film of an element such astantalum (Ta), titanium (Ti), molybdenum (Mo), tungsten (W), chromium(Cr), or silicon (Si); a nitride film containing the aforementionedelement (typically, a tantalum nitride film, a tungsten nitride film, ora titanium nitride film); an alloy film in which the aforementionedelements are combined (typically, a Mo—W alloy or a Mo—Ta alloy); asilicide film containing the aforementioned element (typically, atungsten silicide film or a titanium silicide film); and the like can beused. Note that the aforementioned single film, nitride film, alloyfilm, silicide film, and the like can have a single-layer structure or astacked-layer structure.

An insulating film 110118 can have a single-layer structure or astacked-layer structure of an insulating film containing oxygen ornitrogen, such as silicon oxide (SiOx), silicon nitride (SiNx), siliconoxynitride (SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y); or afilm containing carbon, such as a DLC (diamond-like carbon), by a knownmethod (such as a sputtering method or a plasma CVD method).

An insulating film 110119 can have a single-layer structure or astacked-layer structure of a siloxane resin; an insulating filmcontaining oxygen or nitrogen, such as silicon oxide (SiOx), siliconnitride (SiNx), silicon oxynitride (SiOxNy) (x>y), or silicon nitrideoxide (SiNxOy) (x>y); a film containing carbon, such as a DLC(diamond-like carbon); or an organic material such as epoxy, polyimide,polyamide, polyvinyl phenol, benzocyclobutene, or acrylic. Note that asiloxane resin corresponds to a resin having Si—O—Si bonds. Siloxaneincludes a backbone structure of a bond of silicon (Si) and oxygen (O).As a substituent, an organic group containing at least hydrogen (such asan alkyl group or an aryl group) is used. Alternatively, a fluoro group,or a fluoro group and an organic group containing at least hydrogen canbe used as a substituent. Note that in a semiconductor device applicableto the invention, the insulating film 110119 can be directly provided soas to cover the gate electrode 110117 without provision of theinsulating film 110118.

As a conductive film 110123, a single film of an element such as Al, Ni,C, W, Mo, Ti, Pt, Cu, Ta, Au, or Mn, a nitride film containing theaforementioned element, an alloy film in which the aforementionedelements are combined, a silicide film containing the aforementionedelement, or the like can be used. For example, as an alloy containingthe plurality of elements, an Al alloy containing C and Ti, an Al alloycontaining Ni, an Al alloy containing C and Ni, an Al alloy containing Cand Mn, or the like can be used. Further, when the conductive film has astacked-layer structure, Al can be interposed between Mo, Ti, or thelike; thus, resistance of Al to heat and chemical reaction can beimproved.

Next, characteristics of each structure are described with reference tothe cross-sectional view of the plurality of transistors each having adifferent structure in FIG. 71A.

Reference numeral 110101 denotes a single drain transistor. Since thesingle drain transistor can be formed by a simple method, it isadvantageous in low manufacturing cost and high yield. Here, thesemiconductor layers 110113 and 110115 have different concentrations ofimpurities, and the semiconductor layer 110113 is used as a channelregion and the semiconductor layers 110115 are used as a source regionand a drain region. By controlling the concentration of impurities inthis manner, resistivity of the semiconductor layer can be controlled.Further, an electrical connection state of the semiconductor layer andthe conductive film 110123 can be closer to ohmic contact. Note that asa method of separately forming the semiconductor layers each havingdifferent amount of impurities, a method can be used in which impuritiesare doped in a semiconductor layer using the gate electrode 110117 as amask.

Reference numeral 110102 denotes a transistor in which the gateelectrode 110117 is tapered at an angle of at least certain degrees.Since the transistor can be formed by a simple method, it isadvantageous in low manufacturing cost and high yield. Here, a taperedangle is 45° or more and less than 95°, and preferably 60° or more andless than 95°. Note that the tapered angle may be less than 45°. Thesemiconductor layers 110113, 110114, and 110115 have differentconcentrations of impurities. The semiconductor layer 110113 is used asa channel region, the semiconductor layers 110114 as lightly doped drain(LDD) regions, and the semiconductor layers 110115 as a source regionand a drain region. By controlling the amount of impurities in thismanner, resistivity of the semiconductor layer can be controlled.Further, an electrical connection state of the semiconductor layer andthe conductive film 110123 can be closer to ohmic contact. Moreover,since the transistor includes the LDD regions, a high electric field ishardly applied inside the transistor, so that deterioration of theelement due to hot carriers can be suppressed. Note that as a method ofseparately forming the semiconductor layers having different amount ofimpurities, a method can be used in which impurities are doped in asemiconductor layer using the gate electrode 110117 as a mask. In thetransistor 110102, since the gate electrode 110117 is tapered at anangle of at least certain degrees, gradient of the concentration ofimpurities doped in the semiconductor layer through the gate electrode110117 can be provided, and the LDD region can be easily formed.

Reference numeral 110103 denotes a transistor in which the gateelectrode 110117 is formed of at least two layers and a lower gateelectrode is longer than an upper gate electrode. In this specification,a shape of the lower and upper gate electrodes is called a hat shape.When the gate electrode 110117 has a hat shape, an LDD region can beformed without addition of a photomask. Note that a structure where theLDD region overlaps with the gate electrode 110117, like the transistor110103, is particularly called a GOLD (gate overlapped LDD) structure.As a method of forming the gate electrode 110117 with a hat shape, thefollowing method may be used.

First, when the gate electrode 110117 is patterned, the lower and uppergate electrodes are etched by dry etching so that side surfaces thereofare inclined (tapered). Then, an inclination of the upper gate electrodeis processed to be almost perpendicular by anisotropic etching. Thus,the gate electrode a cross section of which is a hat shape is formed.After that, impurity elements are doped twice, so that the semiconductorlayer 110113 used as the channel region, the semiconductor layers 110114used as the LDD regions, and the semiconductor layers 110115 used as asource electrode and a drain electrode are formed.

Note that part of the LDD region, which overlaps with the gate electrode110117, is referred to as an Lov region, and part of the LDD region,which does not overlap with the gate electrode 110117, is referred to asan Loff region. The Loff region is highly effective in suppressing anoff-current value, whereas it is not very effective in preventingdeterioration in an on-current value due to hot carriers by relieving anelectric field in the vicinity of the drain. On the other hand, the Lovregion is effective in preventing deterioration in the on-current valueby relieving the electric field in the vicinity of the drain, whereas itis not very effective in suppressing the off-current value. Thus, it ispreferable to form a transistor having a structure appropriate forcharacteristics of each of the various circuits. For example, when asemiconductor device applicable to the invention is used for a displaydevice, a transistor having an Loff region is preferably used as a pixeltransistor in order to suppress the off-current value. On the otherhand, as a transistor in a peripheral circuit, a transistor having anLov region is preferably used in order to prevent deterioration in theon-current value by relieving the electric field in the vicinity of thedrain.

Reference numeral 110104 denotes a transistor including a sidewall110121 in contact with the side surface of the gate electrode 110117.When the transistor includes the sidewall 110121, a region overlappingwith the sidewall 110121 can be made to be an LDD region.

Reference numeral 110105 denotes a transistor in which an LDD (Loft)region is formed by doping in the semiconductor layer with the use of amask. Thus, the LDD region can surely be formed, and an off-currentvalue of the transistor can be reduced.

Reference numeral 110106 denotes a transistor in which an LDD (Lov)region is formed by doping in the semiconductor layer with the use of amask. Thus, the LDD region can surely be formed, and deterioration in anon-current value can be prevented by relieving the electric field in thevicinity of the drain of the transistor.

Next, an example of a method for manufacturing a transistor included ina semiconductor device to which the invention can be applied isdescribed with reference to FIGS. 71B to 71G.

Note that a structure and a manufacturing method of a transistorincluded in a semiconductor device to which the invention can be appliedare not limited to those in FIGS. 71A to 71G, and various structures andmanufacturing methods can be used.

In this embodiment mode, a surface of the substrate 110111, a surface ofthe insulating film 110112, a surface of the semiconductor layer 110113,a surface of the semiconductor layer 110114, a surface of thesemiconductor layer 110115, a surface of the insulating film 110116, asurface of the insulating film 110118, or a surface of the insulatingfilm 110119 is oxidized or nitrided by using plasma treatment, so thatthe semiconductor layer or the insulating film can be oxidized ornitrided. By oxidizing or nitriding the semiconductor layer or theinsulating film by plasma treatment in such a manner, the surface of thesemiconductor layer or the insulating film is modified, and theinsulating film can be formed to be denser than an insulating filmformed by a CVD method or a sputtering method. Thus, a defect such as apinhole can be suppressed, and characteristics and the like of asemiconductor device can be improved.

First, the surface of the substrate 110111 is washed using hydrofluoricacid (HF), alkaline, or pure water. The substrate 110111 can be a glasssubstrate using barium borosilicate glass, aluminoborosilicate glass, orthe like, a quartz substrate, a ceramic substrate, a metal substratecontaining stainless steel, or the like. In addition, a substrate formedof plastics typified by polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), or polyethersulfone (PES), or a substrate formed of aflexible synthetic resin such as acrylic can also be used. Here, thecase where a glass substrate is used as the substrate 110111 is shown.

Here, an oxide film or a nitride film may be formed on the surface ofthe substrate 110111 by oxidizing or nitriding the surface of thesubstrate 110111 by plasma treatment (FIG. 71B). Hereinafter, aninsulating film such as an oxide film or a nitride film formed byperforming plasma treatment on the surface is also referred to as aplasma-treated insulating film. In FIG. 71B, an insulating film 110131is a plasma-treated insulating film. In general, when a semiconductorelement such as a thin film transistor is provided over a substrateformed of glass, plastic, or the like, an impurity element such asalkali metal (e.g., Na) or alkaline earth metal included in glass,plastic, or the like might be mixed into the semiconductor element sothat the semiconductor element is contaminated; thus, characteristics ofthe semiconductor element may be adversely affected in some cases.Nitridation of a surface of the substrate formed of glass, plastic, orthe like can prevent an impurity element such as alkali metal (e.g., Na)or alkaline earth metal included in the substrate from being mixed intothe semiconductor element.

When the surface is oxidized by plasma treatment, the plasma treatmentis performed in an oxygen atmosphere (e.g., in an atmosphere of oxygen(O₂) and a rare gas (containing at least one of He, Ne, Ar, Kr, and Xe),in an atmosphere of oxygen, hydrogen (H₂), and a rare gas, or in anatmosphere of dinitrogen monoxide and a rare gas). On the other hand,when the surface is nitrided by plasma treatment, the plasma treatmentis performed in a nitrogen atmosphere (e.g., in an atmosphere ofnitrogen (N₂) and a rare gas (containing at least one of He, Ne, Ar, Kr,and Xe), in an atmosphere of nitrogen, hydrogen, and a rare gas, or inan atmosphere of NH₃ and a rare gas). As a rare gas, Ar can be used, forexample. Alternatively, a gas in which Ar and Kr are mixed may be used.Accordingly, the plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for plasmatreatment. For example, the plasma-treated insulating film contains Arwhen Ar is used in addition, it is preferable to perform plasmatreatment in the atmosphere containing the aforementioned gas, withconditions of an electron density in the range of 1×10¹¹ to 1×10¹³ cm⁻³and a plasma electron temperature in the range of 0.5 to 1.5 eV. Sincethe plasma electron density is high and the electron temperature in thevicinity of an object to be treated is low, damage by plasma to theobject to be treated can be prevented. Further, since the plasmaelectron density is as high as 1×10¹¹ cm³ or more, an oxide film or anitride film formed by oxidizing or nitriding the object to be treatedby plasma treatment is superior in its uniformity of thickness and thelike as well as being dense, as compared with a film formed by a CVDmethod, a sputtering method, or the like. Alternatively, since theplasma electron temperature is as low as 1 eV or less, oxidation ornitridation can be performed at a lower temperature as compared with aconventional plasma treatment or thermal oxidation. For example,oxidation or nitridation can be performed sufficiently even when plasmatreatment is performed at a temperature lower than a strain point of aglass substrate by 100 degrees or more. Note that as frequency forgenerating plasma, high frequency waves such as microwaves (2.45 GHz)can be used. Note that hereinafter, plasma treatment is performed usingthe aforementioned conditions unless otherwise specified.

Although FIG. 71B shows the case where the plasma-treated insulatingfilm is formed by plasma treatment on the surface of the substrate110111, this embodiment mode includes the case where a plasma-treatedinsulating film is not formed on the surface of the substrate 110111.

Although a plasma-treated insulating film formed by plasma treatment onthe surface of the object to be treated is not shown in FIGS. 71C to71G; this embodiment mode includes the case where a plasma-treatedinsulating film formed by plasma treatment exists on the surface of thesubstrate 110111, the insulating film 110112, the semiconductor layer110113, the semiconductor layer 110114, the semiconductor layer 110115,the insulating film 110116, the insulating film 110118, or theinsulating film 110119.

Next, the insulating film 110112 is formed over the substrate 110111 bya known method (such as a sputtering method, an LPCVD method, or aplasma CVD method) (FIG. 71C. For the insulating film 110112, siliconoxide (SiOx) or silicon oxynitride (SiOxNy) (x>y) can be used.

Here, a plasma-treated insulating film may be formed on the surface ofthe insulating film 110112 by oxidizing or nitriding the surface of theinsulating film 110112 by plasma treatment. By oxidizing the surface ofthe insulating film 110112, the surface of the insulating film 110112 ismodified, and a dense film with fewer defects such as a pinhole can beobtained. Further, by oxidizing the surface of the insulating film110112, the plasma-treated insulating film containing a little amount ofN atoms can be formed; thus, interface characteristics of theplasma-treated insulating film and a semiconductor layer are improvedwhen the semiconductor layer is provided over the plasma-treatedinsulating film. The plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for plasmatreatment. Note that the plasma treatment can be similarly performedunder the aforementioned conditions.

Next, the island-shaped semiconductor layers 110113 and 110114 areformed over the insulating film 110112 (FIG. 71D). The island-shapedsemiconductor layers 110113 and 110114 can be formed in such a mannerthat an amorphous semiconductor layer is formed over the insulating film110112 by using a material containing silicon (Si) as its main component(e.g., Si_(X)Ge_(1-x)) or the like by a known method (such as asputtering method, an LPCVD method, or a plasma CVD method), theamorphous semiconductor layer is crystallized, and the semiconductorlayer is selectively etched. Note that crystallization of the amorphoussemiconductor layer can be performed by a known crystallization methodsuch as a laser crystallization method, a thermal crystallization methodusing RTA or an annealing furnace, a thermal crystallization methodusing a metal element which promotes crystallization, or a method inwhich these methods are combined. Here, end portions of theisland-shaped semiconductor layers are provided with an angle of about90° (θ=85 to 100°). Alternatively, the semiconductor layer 110114 to bea low concentration drain region may be formed by doping impurities withthe use of a mask.

Here, a plasma-treated insulating film may be formed on the surfaces ofthe semiconductor layers 110113 and 110114 by oxidizing or nitriding thesurfaces of the semiconductor layers 110113 and 110114 by plasmatreatment. For example, when Si is used for the semiconductor layers110113 and 110114, silicon oxide (SiOx) or silicon nitride (SiNx) isformed as the plasma-treated insulating film. Alternatively, after thesemiconductor layers 110113 and 110114 are oxidized by plasma treatment,the semiconductor layers 110113 and 110114 may be nitrided by performingplasma treatment again. In this case, silicon oxide (SiOx) is formed incontact with the semiconductor layers 110113 and 110114, and siliconnitride oxide (SiNxOy) (x>y) is formed on the surface of the siliconoxide. Note that when the semiconductor layer is oxidized by plasmatreatment, the plasma treatment is performed in an oxygen atmosphere(e.g., in an atmosphere of oxygen (O₂) and a rare gas (containing atleast one of He, Ne, Ar, Kr, and Xe), in an atmosphere of oxygen,hydrogen (H₂), and a rare gas, or in an atmosphere of dinitrogenmonoxide and a rare gas). On the other hand, when the semiconductorlayer is nitrided by plasma treatment, the plasma treatment is performedin a nitrogen atmosphere (e.g., in an atmosphere of nitrogen (N₂) and arare gas (containing at least one of He, Ne, Ar, Kr, and Xe), in anatmosphere of nitrogen, hydrogen, and a rare gas, or in an atmosphere ofNH₃ and a rare gas). As a rare gas, Ar can be used, for exampleAlternatively, a gas in which Ar and Kr are mixed may be used.Accordingly, the plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for plasmatreatment. For example, the plasma-treated insulating film contains Arwhen Ar is used.

Next, the insulating film 110116 is formed (FIG. 71E). The insulatingfilm 110116 can have a single-layer structure or a stacked-layerstructure of an insulating film containing oxygen or nitrogen, such assilicon oxide (SiOx), silicon nitride (SiNx), silicon oxynitride(SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y), by a knownmethod (such as a sputtering method, an LPCVD method, or a plasma CVDmethod). Note that when the plasma-treated insulating film is formed onthe surfaces of the semiconductor layers 110113 and 110114 by plasmatreatment to the surfaces of the semiconductor layers 110113 and 110114,the plasma-treated insulating film can be used as the insulating film110116.

Here, the surface of the insulating film 110116 may be oxidized ornitrided by plasma treatment, so that a plasma-treated insulating filmis formed on the surface of the insulating film 110116. Note that theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for plasma treatment. The plasmatreatment can be similarly performed under the aforementionedconditions.

Alternatively, after the insulating film 110116 is oxidized by plasmatreatment once in an oxygen atmosphere, the insulating film 110116 maybe nitrided by performing plasma treatment again in a nitrogenatmosphere. By oxidizing or nitriding the surface of the insulating film110116 by plasma treatment in such a manner, the surface of theinsulating film 110116 is modified, and a dense film can be formed. Aninsulating film obtained by plasma treatment is denser and has fewerdefects such as a pinhole, as compared with an insulating film formed bya CVD method, a sputtering method, or the like. Thus, characteristics ofa thin film transistor can be improved.

Next, the gate electrode 110117 is formed (FIG. 71F). The gate electrode110117 can be formed by a known method (such as a sputtering method, anLPCVD method, or a plasma CVD method).

In the transistor 110101, the semiconductor layers 110115 used as thesource region and the drain region can be formed by doping impuritiesafter the gate electrode 110117 is formed.

In the transistor 110102, the semiconductor layers 110114 used as theLDD regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 110117 is formed.

In the transistor 110103, the semiconductor layers 110114 used as theLDD regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 110117 is formed.

In the transistor 110104, the semiconductor layers 110114 used as theLDD regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thesidewall 110121 is formed on the side surface of the gate electrode110117.

Note that silicon oxide (SiOx) or silicon nitride (SiNx) can be used forthe sidewall 110121. As a method of forming the sidewall 110121 on theside surface of the gate electrode 110117, a method can be used, forexample, in which a silicon oxide (SiOx) film or a silicon nitride(SiNx) film is formed by a known method after the gate electrode 110117is formed, and then, the silicon oxide (SiOx) film or the siliconnitride (SiNx) film is etched by anisotropic etching. Thus, the siliconoxide (SiOx) film or the silicon nitride (SiNx) film remains only on theside surface of the gate electrode 110117, so that the sidewall 110121can be formed on the side surface of the gate electrode 110117.

In the transistor 110105, the semiconductor layers 110114 used as theLDD (Loff) regions and the semiconductor layer 110115 used as the sourceregion and the drain region can be formed by doping impurities after amask 110122 is formed to cover the gate electrode 110117.

In the transistor 110106, the semiconductor layers 110114 used as theLDD (Lov) regions and the semiconductor layers 110115 used as the sourceregion and the drain region can be formed by doping impurities after thegate electrode 110117 is formed.

Next, the insulating film 110118 is formed (FIG. 71G). The insulatingfilm 110118 can have a single-layer structure or a stacked-layerstructure of an insulating film containing oxygen or nitrogen, such assilicon oxide (SiOx), silicon nitride (SiNx), silicon oxynitride(SiOxNy) (x>y), or silicon nitride oxide (SiNxOy) (x>y); or a filmcontaining carbon, such as a DLC (diamond-like carbon), by a knownmethod (such as a sputtering method or a plasma CVD method).

Here, the surface of the insulating film 110118 may be oxidized ornitrided by plasma treatment, so that a plasma-treated insulating filmis formed on the surface of the insulating film 110118. Note that theplasma-treated insulating film contains a rare gas (containing at leastone of He, Ne, Ar, Kr, and Xe) used for plasma treatment. The plasmatreatment can be similarly performed under the aforementionedconditions.

Next, the insulating film 110119 is formed. The insulating film 110119can have a single-layer structure or a stacked-layer structure of anorganic material such as epoxy, polyimide, polyamide, polyvinyl phenol,benzocyclobutene, or acrylic; or a siloxane resin, in addition to aninsulating film containing oxygen or nitrogen, such as silicon oxide(SiOx), silicon nitride (SiNx), silicon oxynitride (SiOxNy) (x>y), orsilicon nitride oxide (SiNxOy) (x>y); or a film containing carbon, suchas a DLC (diamond-like carbon), by known method (such as a sputteringmethod or a plasma CVD method). Note that a siloxane resin correspondsto a resin having Si—O—Si bonds. Siloxane includes a skeleton structureof a bond of silicon (Si) and oxygen (O). As a substituent, an organicgroup containing at least hydrogen (such as an alkyl group or an arylgroup) is used. Alternatively, a fluoro group, or a fluoro group and anorganic group containing at least hydrogen can be used as a substituent.In addition, the plasma-treated insulating film contains a rare gas(containing at least one of He, Ne, Ar, Kr, and Xe) used for plasmatreatment. For example, the plasma-treated insulating film contains Arwhen Ar is used.

When an organic material such as polyimide, polyamide, polyvinyl phenol,benzocyclobutene, or acrylic, or a siloxane resin is used for theinsulating film 110119, the surface of the insulating film 110119 can bemodified by oxidizing or nitriding the surface of the insulating film byplasma treatment. Modification of the surface improves strength of theinsulating film 110119, and physical damage such as a crack generatedwhen an opening is formed, for example, or film reduction in etching canbe reduced. Further, when the conductive film 110123 is formed over theinsulating film 110119, modification of the surface of the insulatingfilm 110119 improves adhesion to the conductive film. For example, whena siloxane resin is used for the insulating film 110119 and nitrided byplasma treatment, a plasma-treated insulating film containing nitrogenor a rare gas is formed by nitriding a surface of the siloxane resin,and physical strength is improved.

Next, a contact hole is formed in the insulating films 110119, 110118,and 110116 in order to form the conductive film 110123 which iselectrically connected to the semiconductor layer 110115. Note that thecontact hole may have a tapered shape. Thus, coverage with theconductive film 110123 can be improved.

FIG. 75 shows cross-sectional structures of a bottom-gate transistor anda capacitor.

A first insulating film (an insulating film 110502) is formed over anentire surface of a substrate 110501. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

A first conductive layer (conductive layers 110503 and 110504) is formedover the first insulating film. The conductive layer 110503 includes aportion functioning as a gate electrode of a transistor 110520. Theconductive layer 110504 includes a portion functioning as a firstelectrode of a capacitor 110521. As the first conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively,a stacked layer of these elements (including the alloy thereof) can beused.

A second insulating film (an insulating film 110514) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

For a portion of the second insulating film, which is in contact withthe semiconductor layer, a silicon oxide film is preferably used. Thisis because the trap level at the interface between the semiconductorlayer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A semiconductor layer is formed in part of a portion over the secondinsulating film, which overlaps with the first conductive layer, by aphotolithography method, an inkjet method, a printing method, or thelike. Part of the semiconductor layer extends to a portion over thesecond insulating film, which does not overlap with the first conductivelayer. The semiconductor layer includes a channel formation region (achannel formation region 110510), an LDD region (LDD regions 110508 and110509), and an impurity region (impurity regions 110505, 110506, and110507). The channel formation region 110510 functions as a channelformation region of the transistor 110520. The LDD regions 110508 and110509 function as LDD regions of the transistor 110520. Note that theLDD regions 110508 and 110509 are not necessarily formed. The impurityregion 110505 includes a portion functioning as one of a sourceelectrode and a drain electrode of the transistor 110520. The impurityregion 110506 includes a portion functioning as the other of the sourceelectrode and the drain electrode of the transistor 110520. The impurityregion 110507 includes a portion functioning as a second electrode ofthe capacitor 110521.

A third insulating film (an insulating film 110511) is entirely formed.A contact hole is selectively formed in part of the third insulatingfilm. The insulating film 110511 functions as an interlayer film. As thethird insulating film, an inorganic material (e.g., silicon oxide,silicon nitride, or silicon oxynitride), an organic compound materialhaving a low dielectric constant (e.g., a photosensitive ornonphotosensitive organic resin material), or the like can be used.Alternatively, a material containing siloxane may be used. Note thatsiloxane is a material in which a skeleton structure is formed by a bondof silicon (Si) and oxygen (O). As a substitute, an organic groupcontaining at least hydrogen (such as an alkyl group or an aryl group)is used. Alternatively, a fluoro group, or a fluoro group and an organicgroup containing at least hydrogen may be used as a substituent.

A second conductive layer (conductive layers 110512 and 110513) isformed over the third insulating film. The conductive layer 110512 isconnected to the other of the source electrode and the drain electrodeof the transistor 110520 through the contact hole formed in the thirdinsulating film Thus, the conductive layer 110512 includes a portionfunctioning as the other of the source electrode and the drain electrodeof the transistor 110520. When the conductive layer 110513 iselectrically connected to the conductive layer 110504, the conductivelayer 110513 includes a portion functioning as the first electrode ofthe capacitor 110521. As the second conductive layer, an element such asTi, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, oran alloy of these elements can be used. Alternatively, a stacked layerof these elements (including the alloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Next, structures of a transistor and a capacitor are described in thecase where an amorphous silicon (a-Si:H) film is used as a semiconductorlayer of the transistor.

FIG. 72 shows cross-sectional structures of a top-gate transistor and acapacitor.

A first insulating film (an insulating film 110202) is formed over anentire surface of a substrate 110201. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand manufacturing cost can be realized. Further, since the structure canbe simplified, the yield can be improved.

A first conductive layer (conductive layers 110203, 110204, and 110205)is formed over the first insulating film. The conductive layer 110203includes a portion functioning as one of a source electrode and a drainelectrode of a transistor 110220. The conductive layer 110204 includes aportion functioning as the other of the source electrode and the drainelectrode of the transistor 110220. The conductive layer 110205 includesa portion functioning as a first electrode of a capacitor 110221. As thefirst conductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd,Cu, Ag, Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elementscan be used. Alternatively, a stacked layer of these elements (includingthe alloy thereof) can be used.

A first semiconductor layer (semiconductor layers 110206 and 110207) isformed above the conductive layers 110203 and 110204. The semiconductorlayer 110206 includes a portion functioning as one of the sourceelectrode and the drain electrode. The semiconductor layer 110207includes a portion functioning as the other of the source electrode andthe drain electrode. As the first semiconductor layer, siliconcontaining phosphorus or the like can be used, for example.

A second semiconductor layer (a semiconductor layer 110208) is formedover the first insulating film and between the conductive layer 110203and the conductive layer 110204. Part of the semiconductor layer 110208extends over the conductive layers 110203 and 110204. The semiconductorlayer 110208 includes a portion functioning as a channel region of thetransistor 110220. As the second semiconductor layer, a semiconductorlayer having no crystallinity such as an amorphous silicon (a-Si:H)layer, a semiconductor layer such as a microcrystalline semiconductor(μ-Si:H) layer, or the like can be used.

A second insulating film (insulating films 110209 and 110210) is formedto cover at least the semiconductor layer 110208 and the conductivelayer 110205. The second insulating film functions as a gate insulatingfilm. As the second insulating film, a single layer or a stacked layerof a silicon oxide film, a silicon nitride film, a silicon oxynitridefilm (SiOxNy), or the like can be used.

For a portion of, the second insulating film, which is in contact withthe second semiconductor layer, a silicon oxide film is preferably used.This is because the trap level at the interface between the secondsemiconductor layer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A second conductive layer (conductive layers 110211 and 110212) isformed over the second insulating film. The conductive layer 110211includes a portion functioning as a gate electrode of the transistor110220. The conductive layer 110212 functions as a second electrode ofthe capacitor 110221 or a wiring. As the second conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively,a stacked layer of these elements (including the alloy thereof) can beused.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

FIG. 73 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 73 has a channel etch structure.

A first insulating film (an insulating film 110302) is formed over anentire surface of a substrate 110301. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand manufacturing cost can be realized. Further, since the structure canbe simplified, the yield can be improved.

A first conductive layer (conductive layers 110303 and 110304) is formedover the first insulating film. The conductive layer 110303 includes aportion functioning as a gate electrode of a transistor 110320. Theconductive layer 110304 includes a portion functioning as a firstelectrode of a capacitor 110321. As the first conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternatively,a stacked layer of these elements (including the alloy thereof) can beused.

A second insulating film (an insulating film 110305) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

For a portion of the second insulating film, which is in contact withthe semiconductor layer, a silicon oxide film is preferably used. Thisis because the trap level at the interface between the semiconductorlayer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 110306) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer110306 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer110306 includes a portion functioning as a channel region of thetransistor 110320. As the semiconductor layer 110306, a semiconductorlayer having no crystallinity such as an amorphous silicon (a-Si:H)layer, a semiconductor layer such as a microcrystalline semiconductor(μ-Si:H) layer, or the like can be used.

A second semiconductor layer (semiconductor layers 110307 and 110308) isformed over part of the first semiconductor layer. The semiconductorlayer 110307 includes a portion functioning as one of a source electrodeand a drain electrode. The semiconductor layer 110308 includes a portionfunctioning as the other of the source electrode and the drainelectrode. As the second semiconductor layer, silicon containingphosphorus or the like can be used, for example.

A second conductive layer (conductive layers 110309, 110310, and 110311)is formed over the second semiconductor layer and the second insulatingfilm. The conductive layer 110309 includes a portion functioning as oneof the source electrode and the drain electrode of the transistor110320. The conductive layer 110310 includes a portion functioning asthe other of the source electrode and the drain electrode of thetransistor 110320. The conductive layer 110311 includes a portionfunctioning as a second electrode of the capacitor 110321. As the secondconductive layer, an element such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag,Au, Pt, Nb, Si, Zn, Fe, Ba, or Ge, or an alloy of these elements can beused. Alternatively, a stacked layer of these elements (including thealloy thereof) can be used.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a step characteristic of a channel etch typetransistor is described. The first semiconductor layer and the secondsemiconductor layer can be formed using the same mask. Specifically, thefirst semiconductor layer and the second semiconductor layer arecontinuously formed. Further, the first semiconductor layer and thesecond semiconductor layer are formed using the same mask.

Another example of a step characteristic of a channel etch typetransistor is described. The channel region of the transistor can beformed without using an additional mask. Specifically, after the secondconductive layer is formed, part of the second semiconductor layer isremoved using the second conductive layer as a mask. Alternatively, partof the second semiconductor layer is removed by using the same mask asthe second conductive layer. The first semiconductor layer below theremoved second semiconductor layer serves as the channel region of thetransistor.

FIG. 74 shows cross-sectional structures of an inversely staggered(bottom gate) transistor and a capacitor. In particular, the transistorshown in FIG. 74 has a channel protection (channel stop) structure.

A first insulating film (an insulating film 110402) is formed over anentire surface of a substrate 110401. The first insulating film canprevent impurities from the substrate from adversely affecting asemiconductor layer and changing properties of a transistor. That is,the first insulating film functions as a base film. Thus, a transistorwith high reliability can be formed. As the first insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

Note that the first insulating film is not necessarily formed. When thefirst insulating film is not formed, reduction in the number of stepsand manufacturing cost can be realized. Further, since the structure canbe simplified, the yield can be improved.

A first conductive layer (conductive layers 110403 and 110404) is formedover the first insulating film. The conductive layer 110403 includes aportion functioning as a gate electrode of a transistor 110420. Theconductive layer 110404 includes a portion functioning as a firstelectrode of a capacitor 110421. As the first conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternately, astacked layer of these elements (including the alloy thereof) can beused.

A second insulating film (an insulating film 110405) is formed to coverat least the first conductive layer. The second insulating filmfunctions as a gate insulating film. As the second insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

For a portion of the second insulating film, which is in contact withthe semiconductor layer, a silicon oxide film is preferably used. Thisis because the trap level at the interface between the semiconductorlayer and the second insulating film is lowered.

When the second insulating film is in contact with Mo, a silicon oxidefilm is preferably used for a portion of the second insulating film incontact with Mo. This is because the silicon oxide film does not oxidizeMo.

A first semiconductor layer (a semiconductor layer 110406) is formed inpart of a portion over the second insulating film, which overlaps withthe first conductive layer, by a photolithography method, an inkjetmethod, a printing method, or the like. Part of the semiconductor layer110406 extends to a portion over the second insulating film, which doesnot overlap with the first conductive layer. The semiconductor layer110406 includes a portion functioning as a channel region of thetransistor 110420. As the semiconductor layer 110406, a semiconductorlayer having no crystallinity such as an amorphous silicon (a-Si:H)layer, a semiconductor layer such as a microcrystalline semiconductor(μ-Si:H) layer, or the like can be used.

A third insulating film (an insulating film 110412) is formed over partof the first semiconductor layer. The insulating film 110412 preventsthe channel region of the transistor 110420 from being removed byetching. That is, the insulating film 110412 functions as a channelprotection film (a channel stop film). As the third insulating film, asingle layer or a stacked layer of a silicon oxide film, a siliconnitride film, a silicon oxynitride film (SiOxNy), or the like can beused.

A second semiconductor layer (semiconductor layers 110407 and 110408) isformed over part of the first semiconductor layer and part of the thirdinsulating film The semiconductor layer 110407 includes a portionfunctioning as one of a source electrode and a drain electrode. Thesemiconductor layer 110408 includes a portion functioning as the otherof the source electrode and the drain electrode. As the secondsemiconductor layer, silicon containing phosphorus or the like can beused, for example.

A second conductive layer (conductive layers 110409, 110410, and 110411)is formed over the second semiconductor layer. The conductive layer110409 includes a portion functioning as one of the source electrode andthe drain electrode of the transistor 110420. The conductive layer110410 includes a portion functioning as the other of the sourceelectrode and the drain electrode of the transistor 110420. Theconductive layer 110411 includes a portion functioning as a secondelectrode of the capacitor 110421. As the second conductive layer, anelement such as Ti, Mo, Ta, Cr, W, Al, Nd, Cu, Ag, Au, Pt, Nb, Si, Zn,Fe, Ba, or Ge, or an alloy of these elements can be used. Alternately, astacked layer of these elements (including the alloy thereof) can beused.

Note that in steps after forming the second conductive layer, variousinsulating films or various conductive films may be formed.

Here, an example of a step characteristic of a channel protectiontransistor is described. The first semiconductor layer, the secondsemiconductor layer, and the second conductive layer can be formed usingthe same mask. At the same time, the channel region can be formed.Specifically, the first semiconductor layer is formed, and next, thethird insulating film (i.e., the channel protection film or the channelstop film) is patterned using a mask. Next, the second semiconductorlayer and the second conductive layer are continuously formed. Then,after the second conductive layer is formed, the first semiconductorlayer, the second semiconductor layer, and the second conductive filmare patterned using the same mask. Note that part of the firstsemiconductor layer below the third insulating film is protected by thethird insulating film, and thus is not removed by etching. This part (apart of the first semiconductor layer over which the third insulatingfilm is formed) serves as the channel region.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 13

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an inorganic EL element is described.

An inorganic EL element is classified as either a dispersion typeinorganic EL element or a thin-film type inorganic EL element, dependingon its element structure. These elements differ in that the formerincludes an electroluminescent layer in which particles of alight-emitting material are dispersed in a binder, whereas the latterincludes an electroluminescent layer formed of a thin film of alight-emitting material. However, the former and the latter have incommon in that they need electrons accelerated by a high electric field.Note that mechanisms for obtaining light emission are donor-acceptorrecombination light emission which utilizes a donor level and anacceptor level; and localized light emission which utilizes inner-shellelectron transition of a metal ion. In general, donor-acceptorrecombination light emission is employed in dispersion type inorganic ELelements and localized light emission is employed in thin-film typeinorganic EL elements in many cases.

A light-emitting material includes a base material and an impurityelement to be a luminescence center. Light emission of various colorscan be obtained by changing the impurity element to be included. Thelight-emitting material can be formed using various methods, such as asolid phase method or a liquid phase method (a coprecipitation method).Further, a liquid phase method such as a spray pyrolysis method, adouble decomposition method, a method employing precursor pyrolysis, areverse micelle method, a method in which one or more of these methodsare combined with high-temperature baking, or a freeze-drying method, orthe like can be used.

A solid phase method is a method in which a base material and animpurity element or a compound containing an impurity element areweighed, mixed in a mortar, and heated and baked in an electric furnaceso as to be reacted; thus, the impurity element is included in the basematerial. The baking temperature is preferably 700 to 1500° C. This isbecause a solid-phase reaction does not proceed when the temperature istoo low, and the base material decomposes when the temperature is toohigh. Note that although the materials may be baked in powder form, theyare preferably baked in pellet form. Although a solid phase method needsa comparatively high temperature, it is a simple method, and thus hashigh productivity and is suitable for mass production.

A liquid phase method (a coprecipitation method) is a method in which abase material or a compound containing a base material, and an impurityelement or a compound containing an impurity element are reacted in asolution, dried, and then baked. Particles of a light-emitting materialare uniformly distributed, and the reaction can progress even when theparticles are small and the baking temperature is low.

As a base material to be used for a light-emitting material, sulfide,oxide, or nitride can be used. As sulfide, zinc sulfide (ZnS), cadmiumsulfide (CdS), calcium sulfide (CaS), yttrium sulfide (Y₂S₃), galliumsulfide (Ga₂S₃), strontium sulfide (SrS), barium sulfide (BaS), or thelike can be used, for example. As oxide, zinc oxide (ZnO), yttrium oxide(Y₂O₃), or the like can be used, for example. As nitride, aluminumnitride (AlN), gallium nitride (GaN), indium nitride (InN), or the likecan be used, for example. Further, zinc selenide (ZnSe), zinc telluride(ZnTe), or the like; or a ternary mixed crystal such as calcium galliumsulfide (CaGa₂S₄), strontium gallium sulfide (SrGa₂S₄), or bariumgallium sulfide (BaGa₂S₄) may be used.

As a luminescence center for localized light emission, manganese (Mn),copper (Cu), samarium (Sm), terbium (Tb), erbium (Er), thulium (Tm),europium (Eu), cerium (Ce), praseodymium (Pr), or the like can be used.Note that a halogen element such as fluorine (F) or chlorine (Cl) may beadded for charge compensation.

On the other hand, as a luminescence center for donor-acceptorrecombination light emission, a light-emitting material including afirst impurity element forming a donor level and a second impurityelement forming an acceptor level can be used. As the first impurityelement, fluorine (F), chlorine (Cl), aluminum (Al), or the like can beused, for example. As the second impurity element, copper (Cu), silver(Ag), or the like can be used, for example.

When the light-emitting material for donor-acceptor recombination lightemission is synthesized using a solid phase method, a base material, thefirst impurity element or a compound containing the first impurityelement, and the second impurity element or a compound containing thesecond impurity element are weighed, mixed in a mortar, and heated andbaked in an electric furnace. As the base material, the aforementionedbase material can be used. As the first impurity element or the compoundcontaining the first impurity element, fluorine (F), chlorine (Cl),aluminum sulfide (Al₂S₃), or the like can be used, for example. As thesecond impurity element or the compound containing the second impurityelement, copper (Cu), silver (Ag), copper sulfide (Cu₂S), silver sulfide(Ag₂S), or the like can be used, for example. The baking temperature ispreferably 700 to 1500° C. This is because a solid-phase reaction doesnot proceed when the temperature is too low, and the base materialdecomposes when the temperature is too high. Note that although thematerials may be baked in powder form, they are preferably baked inpellet form.

As the impurity element in the case of using a solid phase reaction, acompound formed of the first impurity element and the second impurityelement may be used in combination. In this case, the impurity elementsare easily diffused, and the solid phase reaction proceeds readily, sothat a uniform light-emitting material can be obtained. Further, sincean unnecessary impurity element is not included, a light-emittingmaterial with high purity can be obtained. As the compound formed of thefirst impurity element and the second impurity element, copper chloride(CuCl), silver chloride (AgCl), or the like can be used, for example.

Note that the concentration of these impurity elements is in the rangeof 0.01 to 10 atomic %, and is preferably in the range of 0.05 to 5atomic % with respect to the base material.

In the case of a thin-film type inorganic EL element, anelectroluminescent layer includes the aforementioned light-emittingmaterial, and can be formed using a vacuum evaporation method such as aresistance heating evaporation method or an electron beam evaporation(EB evaporation) method, a physical vapor deposition (PVD) method suchas a sputtering method, a chemical vapor deposition (CVD) method such asa metal organic CVD method or a low-pressure hydride transport CVDmethod, an atomic layer epitaxy (ALE) method, or the like.

FIGS. 76A to 76C each show an example of a thin-film type inorganic ELelement which can be used as the light-emitting element. In FIGS. 76A to76C, a light-emitting element includes a first electrode layer 120100,an electroluminescent layer 120102, and a second electrode layer 120103.

The light-emitting elements in FIGS. 76B and 76C each have a structurewhere an insulating film is provided between the electrode layer and theelectroluminescent layer in the light-emitting element in FIG. 76A. Thelight-emitting element in FIG. 76B includes an insulating film 120104between the first electrode layer 120100 and the electroluminescentlayer 120102. The light-emitting element in FIG. 76C includes aninsulating film 120105 between the first electrode layer 120100 and theelectroluminescent layer 120102, and an insulating film 120106 betweenthe second electrode layer 120103 and the electroluminescent layer120102. Accordingly, the insulating film may be provided between theelectroluminescent layer and one of the electrode layers interposing theelectroluminescent layer, or may be provided between theelectroluminescent layer and each of the electrode layers interposingthe electroluminescent layer. The insulating film may be a single layeror stacked layers including a plurality of layers.

Note that the insulating film 120104 is provided in contact with thefirst electrode layer 120100 in FIG. 76B; however, the insulating film120104 may be provided in contact with the second electrode layer 120103by reversing the order of the insulating film and the electroluminescentlayer.

In the case of a dispersion type inorganic EL, a film-shapedelectroluminescent layer is formed by dispersing particulatelight-emitting materials in a binder. When particles with a desired sizecannot be sufficiently obtained by a method of forming thelight-emitting material, the light-emitting materials may be processedinto particles by being crushed in a mortar or the like. The binder is asubstance for fixing the particulate light-emitting material in adispersed state and maintaining the shape as the electroluminescentlayer. The light-emitting material is uniformly dispersed in theelectroluminescent layer and fixed by the binder.

In the case of a dispersion type inorganic EL, as a method of formingthe electroluminescent layer, a droplet discharging method by which theelectroluminescent layer can be selectively formed, a printing method(such as screen printing or offset printing), a coating method such as aspin coating method, a dipping method, a dispenser method, or the likecan be used. The thickness of the electroluminescent layer is notparticularly limited, but preferably in the range of 10 to 1000 nm. Inthe electroluminescent layer including the light-emitting material andthe binder, a ratio of the light-emitting material is preferably 50 wt %or more and 80 wt % or less.

FIGS. 77A to 77C each show an example of a dispersion type inorganic ELelement which can be used as the light-emitting element. Alight-emitting element in FIG. 77A has a stacked-layer structure of afirst electrode layer 120200, an electroluminescent layer 120202, and asecond electrode layer 120203. The electroluminescent layer 120202includes a light-emitting material 120201 held by a binder.

An insulating material is used for the binder. As the insulatingmaterial, an organic material or an inorganic material can be used.Alternatively, a mixed material containing an organic material and aninorganic material may be used. As the organic insulating material, apolymer having a comparatively high dielectric constant, such as acyanoethyl cellulose based resin, or a resin such as polyethylene,polypropylene, a polystyrene based resin, a silicone resin, an epoxyresin, or vinylidene fluoride can be used. Alternatively, aheat-resistant polymer such as aromatic polyamide or polybenzimidazole,or a siloxane resin may be used. Note that a siloxane resin correspondsto a resin having Si—O—Si bonds. Siloxane includes a backbone structureof a bond of silicon (Si) and oxygen (O). As a substituent, an organicgroup containing at least hydrogen (such as an alkyl group or an arylgroup) is used. Alternatively, a fluoro group, or a fluoro group and anorganic group containing at least hydrogen may be used as a substituent.Further alternately, a resin material, for example, a vinyl resin suchas polyvinyl alcohol or polyvinylbutyral, a phenol resin, a novolacresin, an acrylic resin, a melamine resin, an urethane resin, an oxazoleresin (polybenzoxazole), or the like may be used. A dielectric constantcan be adjusted by appropriately mixing these resins with fine particleshaving a high dielectric constant, such as barium titanate (BaTiO₃) orstrontium titanate (SrTiO₃).

The inorganic insulating material included in the binder can be formedusing silicon oxide (SiOx), silicon nitride (SiNx), silicon containingoxygen and nitrogen, aluminum nitride (AlN), aluminum containing oxygenand nitrogen, aluminum oxide (Al₂O₃) containing oxygen and nitrogen,titanium oxide (TiO₂), BaTiO₃, SrTiO₃, lead titanate (PbTiO₃), potassiumniobate (KNbO₃), lead niobate (PbNbO₃), tantalum oxide (Ta₂O₅), bariumtantalite (BaTa₂O₆), lithium tantalite (LiTaO₃), yttrium oxide (Y₂O₃),zirconium oxide (ZrO₂), ZnS, or a substance containing another inorganicinsulating material. When an inorganic material having a high dielectricconstant is included in the organic material (by addition or the like),the dielectric constant of the electroluminescent layer formed of thelight-emitting material and the binder can be more effectivelycontrolled, and the dielectric constant can be further increased.

In a manufacturing step, the light-emitting material is dispersed in asolution containing the binder. As a solvent for the solution containingthe binder, it is acceptable as long as a solvent dissolves a bindermaterial and can make a solution having a viscosity suitable for amethod of forming the electroluminescent layer (various wet processes)and for desired film thickness. For example, an organic solvent or thelike can be used as the solvent. When a siloxane resin is used as thebinder, propylene glycol monomethyl ether, propylene glycol monomethylether acetate (also referred to as PGMEA), 3-methoxy-3-methyl-1-butanol(also referred to as MMB), or the like can be used as the solvent.

The light-emitting elements shown in FIGS. 77B and 77C each have astructure where an insulating film is provided between the electrodelayer and the electroluminescent layer in the light-emitting element inFIG. 77A. The light-emitting element in FIG. 77B includes an insulatingfilm 120204 between the first electrode layer 120200 and theelectroluminescent layer 120202. The light-emitting element in FIG. 77Cincludes an insulating film 120205 between the first electrode layer120200 and the electroluminescent layer 120202, and an insulating film120206 between the second electrode layer 120203 and theelectroluminescent layer 120202. Accordingly, the insulating film may beprovided between the electroluminescent layer and one of the electrodelayers interposing the electroluminescent layer, or may be providedbetween the electroluminescent layer and each of the electrode layersinterposing the electroluminescent layer. The insulating film may be asingle layer or stacked layers including a plurality of layers.

Although the insulating film 120204 is provided in contact with thefirst electrode layer 120200 in FIG. 77B, the insulating film 120204 maybe provided in contact with the second electrode layer 120203 byreversing the order of the insulating film and the electroluminescentlayer.

A material used for an insulating film such as the insulating film120104 in FIG. 76B and the insulating film 120204 in FIG. 77B preferablyhas high withstand voltage and dense film quality. Further, the materialpreferably has a high dielectric constant. For example, silicon oxide(SiO₂), yttrium oxide (Y₂O₃), titanium oxide (TiO₂), aluminum oxide(Al₂O₃), hafnium oxide (HfO₂), tantalum oxide (Ta₂O₅), barium titanate(BaTiO₃), strontium titanate (SrTiO₃), lead titanate (PbTiO₃), siliconnitride (Si₃N₄), zirconium oxide (ZrO₂), or the like; or a mixed film ofthose materials or a stacked-layer film including two or more of thosematerials can be used. The insulating film can be formed by sputtering,evaporation, CVD, or the like. The insulating film may be formed bydispersing particles of the insulating material in a binder. A bindermaterial may be formed using a material and a method similar to those ofthe binder contained in the electroluminescent layer. The thickness ofthe insulating film is not particularly limited, but preferably in therange of 10 to 1000 nm.

Note that the light-emitting element can emit light when voltage isapplied between the pair of electrode layers interposing theelectroluminescent layer. The light-emitting element can operate with DCdrive or AC drive.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 14

In this embodiment mode, an example of a display device is described. Inparticular, the case where a display device is optically treated isdescribed.

A rear projection display device 130100 in FIGS. 78A and 78B is providedwith a projector unit 130111, a mirror 130112, and a screen panel130101. The rear projection display device 130100 may also be providedwith a speaker 130102 and operation switches 130104. The projector unit130111 is provided at a lower portion of a housing 130110 of the rearprojection display device 130100, and projects incident light whichprojects an image based on a video signal to the mirror 130112. The rearprojection display device 130100 displays an image projected from a rearsurface of the screen panel 130101.

FIG. 79 shows a front projection display device 130200. The frontprojection display device 130200 is provided with the projector unit130111 and a projection optical system 130201. The projection opticalsystem 130201 projects an image to a screen or the like provided at thefront.

Hereinafter, a structure of the projector unit 130111 which is appliedto the rear projection display device 130100 in FIGS. 78A and 78B andthe front projection display device 130200 in FIG. 79 is described.

FIG. 80 shows a structure example of the projector unit 130111. Theprojector unit 130111 is provided with a light source unit 130301 and amodulation unit 130304. The light source unit 130301 is provided with alight source optical system 130303 including lenses and a light sourcelamp 130302. The light source lamp 130302 is stored in a housing so thatstray light is not scattered. As the light source lamp 130302, ahigh-pressure mercury lamp or a xenon lamp, for example, which can emita large amount of light is used. The light source optical system 130303is provided with an optical lens, a film having a function of polarizinglight, a film for adjusting phase difference, an IR film, or the like asappropriate. The light source unit 130301 is provided so that emittedlight is incident on the modulation unit 130304. The modulation unit130304 is provided with a plurality of display panels 130308, a colorfilter, a dichroic mirror 130305, a total reflection mirror 130306, aretardation plate 130307, a prism 130309, and a projection opticalsystem 130310. Light emitted from the light source unit 130301 is splitinto a plurality of optical paths by the dichroic mirror 130305.

Each optical path is provided with the display panel 130308 and a colorfilter which transmits light with a predetermined wavelength orwavelength range. The transmissive display panel 130308 modulatestransmitted light based on a video signal. Light of each colortransmitted through the display panel 130308 is incident on the prism130309, and an image is displayed on a screen through the projectionoptical system 130310. Note that a Fresnel lens may be provided betweenthe mirror and the screen. Projected light which is projected by theprojector unit 130111 and reflected by the mirror is converted intogenerally parallel light by the Fresnel lens to be projected on thescreen. Displacement between a chief ray and an optical axis of theparallel light is preferably ±10° or less, and more preferably, ±5° orless.

FIG. 81 shows the projector unit 130111 provided with reflective displaypanels 130407, 130408, and 130409. Reference Numeral 130410 is a prism.

The projector unit 130111 in FIG. 81 is provided with the light sourceunit 130101 and a modulation unit 130400. The light source unit 130101may have a structure similar to FIG. 80. Light from the light sourceunit 130101 is split into a plurality of optical paths by dichroicmirrors 130401 and 130402 and a total reflection mirror 130403 to beincident on polarization beam splitters 130404, 130405, and 130406. Thepolarization beam splitters 130404, 130405, and 130406 are providedcorresponding to the reflective display panels 130407, 130408, and130409 which correspond to respective colors. The reflective displaypanels 130407, 130408, and 130409 modulate reflected light based on avideo signal. Light of each color which is reflected by the reflectivedisplay panels 130407, 130408, and 130409 is incident on the prism130109 to be synthesized, and projected through a projection opticalsystem 130411.

Among light emitted from the light source unit 130101, only light in awavelength region of red is transmitted through the dichroic mirror130401 and light in wavelength regions of green and blue is reflected bythe dichroic mirror 130401. Further, only the light in the wavelengthregion of green is reflected by the dichroic mirror 130402. The light inthe wavelength region of red, which is transmitted through the dichroicmirror 130401, is reflected by the total reflection mirror 130403 andincident on the polarization beam splitter 130404. The light in thewavelength region of blue is incident on the polarization beam splitter130405. The light in the wavelength region of green is incident on thepolarization beam splitter 130406. The polarization beam splitters130404, 130405, and 130406 have a function of splitting incident lightinto p-polarized light and s-polarized light and a function oftransmitting only p-polarized light. The reflective display panels130407, 130408, and 130409 polarize incident light based on a videosignal.

Only s-polarized light corresponding to each color is incident on thereflective display panels 130407, 130408, and 130409 corresponding toeach color. Note that the reflective display panels 130407, 130408, and130409 may be liquid crystal panels. In this case, the liquid crystalpanel operates in an electrically controlled birefringence (ECB) mode.Liquid crystal molecules are vertically aligned with respect to asubstrate at a certain angle. Accordingly, in the reflective displaypanels 130407, 130408, and 130409, when a pixel is in an off state,display molecules are aligned so as not to change a polarization stateof incident light but to reflect the incident light. When the pixel isin an on state, alignment of the display molecules is changed, and thepolarization state of the incident light is changed.

The projector unit 130111 in FIG. 81 can be applied to the rearprojection display device 130100 in FIGS. 78A and 78B and the frontprojection display device 130200 in FIG. 79.

FIGS. 82A to 82C show single-panel type projector units. The projectorunit shown in FIG. 82A is provided with the light source unit 130301, adisplay panel 130507, a projection optical system 130511, and aretardation plate 130504. The projection optical system 130511 includesone or a plurality of lenses. The display panel 130507 may be providedwith a color filter.

FIG. 82B shows a structure of a projector unit operating in a fieldsequential mode. A field sequential mode refers to a mode in which colordisplay is performed by light of respective colors such as red, green,and blue sequentially incident on a display panel with a time lag,without a color filter. High-definition image can be displayedparticularly by combination with a display panel with high-speedresponse to change in input signal. In FIG. 82B, a rotating color filterplate 130505 including a plurality of color filters with red, green,blue, or the like is provided between the light source unit 130301 and adisplay panel 130508.

FIG. 82C shows a structure of a projector unit with a color separationmethod using a micro lens, as a color display method. This methodcorresponds to a method in which color display is realized by providinga micro lens array 130506 on a light incident side of a display panel130509 and light of each color is lit from each direction. The projectorunit employing this method has little loss of light due to a colorfilter, so that light from the light source unit 130301 can beefficiently utilized. The projector unit in FIG. 82C is provided withdichroic mirrors 130501, 130502, and 130503 so that light of each coloris lit to the display panel 130509 from each direction.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 15

In this embodiment mode, an operation of a display device is described.

FIG. 83 shows a structure example of a display device.

A display device 180100 includes a pixel portion 180101, a signal linedriver circuit 180103, and a scan line driver circuit 180104. In thepixel portion 180101, a plurality of signal lines S1 to Sn extend fromthe signal line driver circuit 180103 in a column direction. In thepixel portion 180101, a plurality of scan lines G1 to Gm extend from thescan line driver circuit 180104 in a row direction. Pixels 180102 arearranged in matrix at each intersection of the plurality of signal linesS1 to Sn and the plurality of scan lines G1 to Gm.

The signal line driver circuit 180103 has a function of outputting asignal to each of the signal lines S1 to Sn. This signal may be referredto as a video signal. The scan line driver circuit 180104 has a functionof outputting a signal to each of the scan lines G1 to Gm. This signalmay be referred to as a scan signal.

The pixel 180102 includes at least a switching element connected to thesignal line. On/off of the switching element is controlled by apotential of a scan line (a scan signal). When the switching element isturned on, the pixel 180102 is selected. On the other hand, when theswitching element is turned off, the pixel 180102 is not selected.

When the pixel 180102 is selected (a selection state), a video signal isinput to the pixel 180102 from the signal line. A state (e.g.,luminance, transmittance, or voltage of a storage capacitor) of thepixel 180102 is changed in accordance with the video signal input.

When the pixel 180102 is not selected (a non-selection state), the videosignal is not input to the pixel 180102. Note that the pixel 180102holds a potential corresponding to the video signal which is input whenselected; thus, the pixel 180102 maintains the state (e.g., luminance,transmittance, or voltage of a storage capacitor) in accordance with thevideo signal.

Note that a structure of the display device is not limited to that shownin FIG. 83. For example, an additional wiring (such as a scan line, asignal line, a power supply line, a capacitor line, or a common line)may be added in accordance with the structure of the pixel 180102. Asanother example, a circuit having various functions may be added.

FIG. 84 shows an example of a timing chart for describing an operationof a display device.

The timing chart of FIG. 84 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but one frame period is preferably 1/60 second orless so that a viewer does not perceive a flicker.

The timing chart of FIG. 84 shows timing for selecting the scan line G1in the first row, the scan line Gi (one of the scan lines G1 to Gm) inthe i-th row, the scan line Gi+1 in the (i+1)th row, and the scan lineGm in the m-th row.

At the same time as the scan line is selected, the pixel 180102connected to the scan line is also selected. For example, when the scanline Gi in the i-th row is selected, the pixel 180102 connected to thescan line Gi in the i-th row is also selected.

The scan lines G1 to Gm are sequentially selected (hereinafter alsoreferred to as scanned) from the scan line G1 in the first row to thescan line Gm in the m-th row. For example, while the scan line Gi in thei-th row is selected, the scan lines (G1 to Gi−1 and Gi+1 to Gm) otherthan the scan line Gi in the i-th row are not selected. Then, during thenext period, the scan line Gi+1 in the (i+1)th row is selected. Notethat a period during which one scan line is selected is referred to asone gate selection period.

Accordingly, when a scan line in a certain row is selected, videosignals from the signal lines S1 to Sn are input to a plurality ofpixels 180102 connected to the scan line, respectively. For example,while the scan line Gi in the i-th row is selected, given video signalsfrom the signal lines S1 to Sn are input to the plurality of pixels180102 connected to the scan line Gi in the i-th row, respectively.Thus, each of the plurality of pixels 180102 can be controlledindividually by the scan signal and the video signal.

Next, the case where one gate selection period is divided into aplurality of subgate selection periods is described.

FIG. 85 is a timing chart in the case where one gate selection period isdivided into two subgate selection periods (a first subgate selectionperiod and a second subgate selection period).

Note that one gate selection period may be divided into three or moresubgate selection periods.

The timing chart of FIG. 85 shows one frame period corresponding to aperiod when an image of one screen is displayed. One frame period is notparticularly limited, but one frame period is preferably 1/60 second orless so that a viewer does not perceive a flicker.

One frame is divided into two subframes (a first subframe and a secondsubframe).

The timing chart of FIG. 85 shows timing for selecting the scan line Giin the i-th row, the scan line Gi+1 in the (i+1)th row, the scan line Gj(one of the scan lines Gi+1 to Gm) in the j-th row, and the scan lineGj+1 in the (j+1)th row.

At the same time as the scan line is selected, the pixel 180102connected to the scan line is also selected. For example, when the scanline Gi in the i-th row is selected, the pixel 180102 connected to thescan line Gi in the i-th row is also selected.

The scan lines G1 to Gm are sequentially scanned in each subgateselection period. For example, in one gate selection period, the scanline Gi in the i-th row is selected in the first subgate selectionperiod, and the scan line Gj in the j-th row is selected in the secondsubgate selection period. Thus, in one gate selection period, anoperation can be performed as if the scan signals of two rows areselected. At this time, different video signals are input to the signallines S1 to Sn in the first subgate selection period and the secondsubgate selection period. Accordingly, different video signals can beinput to a plurality of pixels 180102 connected to the i-th row and aplurality of pixels 180102 connected to the j-th row.

Next, a driving method for displaying images with high quality isdescribed.

FIGS. 86A and 86B are views for describing high frequency driving.

FIG. 86A shows the case where one image and one intermediate image aredisplayed in one frame period 180400. Reference numerals 180401, 180402,180403, and 180404 denote an image of one frame, an intermediate imageof the frame, an image of the next frame, and an intermediate image ofthe next frame, respectively.

The intermediate image 180402 of the frame may be made based on imagesignals of the frame and the next frame. Alternatively, the intermediateimage 180402 of the frame may be formed from the image 180401 of theframe, or may be a black image. Accordingly, the quality of a movingimage in a hold-type display device can be improved. Further, when oneimage and one intermediate image are displayed in one frame period180400, there is an advantage in that consistency with a frame rate ofthe video signal can be easily obtained and an image processing circuitis not complicated.

FIG. 86B shows the case where one image and two intermediate images aredisplayed in a period with two successive one frame periods 180400 (twoframe periods). Reference numeral 180411, 180412, 180413, and 180414denote an image of the frame, an intermediate image of the frame, anintermediate image of the next frame, an image of a frame after next,respectively.

Each of the intermediate image 180412 of the frame and the intermediateimage 180413 of the next frame may be made based on video signals of theframe, the next frame, and the frame after next. Alternatively, each ofthe intermediate image 180412 of the frame and the intermediate image180413 of the next frame may be a black image. When one image and twointermediate images are displayed in two frame periods, there is anadvantage in that operating frequency of a peripheral driver circuit isnot so high and image quality of a moving image can be effectivelyimproved.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 16

In this embodiment mode, a structure of an EL element is described. Inparticular, a structure of an organic EL element is described.

A structure of a mixed junction EL element is described. As an example,a structure is described, which includes a layer (a mixed layer) inwhich a plurality of materials among a hole injecting material, a holetransporting material, a light-emitting material, an electrontransporting material, an electron injecting material, and the like aremixed (hereinafter referred to as a mixed junction type EL element),which is different from a stacked-layer structure where a hole injectinglayer formed of a hole injecting material, a hole transporting layerformed of a hole transporting material, a light-emitting layer formed ofa light-emitting material, an electron transporting layer formed of anelectron transporting material, an electron injecting layer formed of anelectron injecting material, and the like are clearly distinguished.

FIGS. 87A to 87E are schematic views each showing a structure of a mixedjunction type EL element. Note that a layer interposed between an anode190101 and a cathode 190102 corresponds to an EL layer.

FIG. 87A shows a structure in which an EL layer includes a holetransporting region 190103 formed of a hole transporting material and anelectron transporting region 190104 formed of an electron transportingmaterial. The hole transporting region 190103 is closer to the anodethan the electron transporting region 190104. A mixed region 190105including both the hole transporting material and the electrontransporting material is provided between the hole transporting region190103 and the electron transporting region 190104.

In a direction from the anode 190101 to the cathode 190102, aconcentration of the hole transporting material in the mixed region190105 is decreased and a concentration of the electron transportingmaterial in the mixed region 190105 is increased.

A concentration gradient can be freely set. For example, a ratio ofconcentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material. Alternatively, aratio of concentrations of each functional material may be changed (aconcentration gradient may be formed) in the mixed region 190105including both the hole transporting material and the electrontransporting material, without including the hole transporting layer190103 formed of only the hole transporting material and the electrontransporting layer 190104 formed of only the electron transportingmaterial. Further alternatively, a ratio of concentrations may bechanged depending on a distance from the anode or the cathode. Note thatthe ratio of concentrations may be changed continuously.

A region 190106 to which a light-emitting material is added is includedin the mixed region 190105. A light emission color of the EL element canbe controlled by the light-emitting material. Further, carriers can betrapped by the light-emitting material. As the light-emitting material,various fluorescent dyes as well as a metal complex having a quinolinebackbone, a benzoxazole backbone, or a benzothiazole backbone can beused. The light emission color of the EL element can be controlled byadding the light-emitting material.

As the anode 190101, an electrode material having a high work functionis preferably used in order to inject holes efficiently. For example, atransparent electrode formed of indium tin oxide (ITO), indium zincoxide (IZO), ZnO, SnO₂, In₂O₃, or the like can be used. When atransparency is not needed, the anode 190101 may be formed of an opaquemetal material.

As the hole transporting material, an aromatic amine compound or thelike can be used.

As the electron transporting material, a metal complex having aquinoline derivative, 8-quinolinol, or a derivative thereof as a ligand(especially tris(8-quinolinolato)aluminum (Alq₃)), or the like can beused.

As the cathode 190102, an electrode material having a low work functionis preferably used in order to inject electrons efficiently. A metalsuch as aluminum, indium, magnesium, silver, calcium, barium, or lithiumcan be used by itself. Alternatively, an alloy of the aforementionedmetal or an alloy of the aforementioned metal and another metal may beused.

FIG. 87B is the schematic view of the structure of the EL element, whichis different from that of FIG. 87A. Note that the same portions as thosein FIG. 87A are denoted by the same reference numerals, and descriptionthereof is omitted.

In FIG. 87B, a region to which a light-emitting material is added is notincluded. However, when a material (electron-transporting andlight-emitting material) having both an electron transporting propertyand a light-emitting property, for example,tris(8-quinolinolato)aluminum (Alq₃) is used as a material added to theelectron transporting region 190104, light emission can be performed.

Alternatively, as a material added to the hole transporting region190103, a material (a hole-transporting and light-emitting material)having both a hole transporting property and a light-emitting propertymay be used.

FIG. 87C is the schematic view of the structure of the EL element, whichis different from those of FIGS. 87A and 87B. Note that the sameportions as those in FIGS. 87A and 87B are denoted by the same referencenumerals, and description thereof is omitted.

In FIG. 87C, a region 190107 included in the mixed region 190105 isprovided, to which a hole blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the hole transporting material isadded. The region 190107 to which the hole blocking material is added isprovided closer to the cathode 190102 than the region 190106 in themixed region 190105, to which the light-emitting material is added;thus, a recombination rate of carriers can be increased, and lightemission efficiency can be increased. The structure provided with theregion 190107 to which the hole blocking material is added is especiallyeffective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 87D is the schematic view of the structure of the EL element, whichis different from those of FIGS. 87A to 87C. Note that the same portionsas those in FIGS. 87A to 87C are denoted by the same reference numerals,and description thereof is omitted.

In FIG. 87D, a region 190108 included in the mixed region 190105 isprovided, to which an electron blocking material having a larger energydifference between the highest occupied molecular orbital and the lowestunoccupied molecular orbital than the electron transporting material isadded. The region 190108 to which the electron blocking material isadded is provided closer to the anode 190101 than the region 190106 inthe mixed region 190105, to which the light-emitting material is added;thus, a recombination rate of carriers can be increased, and lightemission efficiency can be increased. The structure provided with theregion 190108 to which the electron blocking material is added isespecially effective in an EL element which utilizes light emission(phosphorescence) by a triplet exciton.

FIG. 87E is the schematic view of the structure of the mixed junctiontype EL element, which is different from those of FIGS. 87A to 87D. FIG.87E shows an example of a structure where a region 190109 to which ametal material is added is included in part of an EL layer in contactwith an electrode of the EL element. In FIG. 87E, the same portions asthose in FIGS. 87A to 87D are denoted by the same reference numerals,and description thereof is omitted. In the structure shown in FIG. 87E,MgAg (an Mg—Ag alloy) may be used as the cathode 190102, and the region190109 to which an Al (aluminum) alloy is added may be included in aregion of the electron transporting region 190104 to which the electrontransporting material is added, which is in contact with the cathode190102, for example. By the aforementioned structure, oxidation of thecathode can be prevented, and electron injection efficiency from thecathode can be increased. Accordingly, the lifetime of the mixedjunction type EL element can be extended. Further, driving voltage canbe lowered.

As a method of forming the mixed junction type EL element, aco-evaporation method or the like can be used.

In the mixed junction type EL elements as shown in FIGS. 87A to 87E, aclear interface between the layers does not exist, and chargeaccumulation can be reduced. Accordingly, the lifetime of the EL elementcan be extended, and a driving voltage can be lowered.

Note that the structures shown in FIGS. 87A to 87E can be implemented infree combination with each other.

A structure of the mixed junction type EL element is not limited tothose described above. A known structure can be freely used.

An organic material which forms an EL layer of an EL element may be alow molecular material or a high molecular material. Alternatively, bothof the materials may be used. When a low molecular material is used asan organic compound material, a film can be formed by an evaporationmethod. When a high molecular material is used as the EL layer, the highmolecular material is dissolved in a solvent and a film can be formed bya spin coating method or an inkjet method.

The EL layer may be formed of a middle molecular material. In thisspecification, a middle molecule organic light-emitting material refersto an organic light-emitting material without a sublimation property andwith a polymerization degree of approximately 20 or less. When a middlemolecular material is used as the EL layer, a film can be formed by aninkjet method or the like.

Note that a low molecular material, a high molecular material, and amiddle molecular material may be used in combination.

An EL element may utilize either light emission (fluorescence) by asinglet exciton or light emission (phosphorescence) by a tripletexciton.

Next, an evaporation device for forming a display device applicable tothe invention is described with reference to drawings.

A display device applicable to the invention may be manufactured toinclude an EL layer. The EL layer is formed including at least partiallya material which exhibits electroluminescence. The EL layer may beformed of a plurality of layers having different functions. In thiscase, the EL layer may be formed of a combination of layers havingdifferent functions, which are also called a hole injecting andtransporting layer, a light-emitting layer, an electron injecting andtransporting layer, and the like.

FIG. 88 shows a structure of an evaporation apparatus for forming an ELlayer over an element substrate provided with a transistor. In theevaporation apparatus, a plurality of treatment chambers are connectedto transfer chambers 190260 and 190261. Each treatment chamber includesa loading chamber 190262 for supplying a substrate, an unloading chamber190263 for collecting the substrate, a heat treatment chamber 190268, aplasma treatment chamber 190272, deposition treatment chambers 190269 to190271, 190273 to 190275 for depositing an EL material, and a depositiontreatment chamber 190276 for forming a conductive film formed ofaluminum or containing aluminum as its main component as one electrodeof an EL element. Gate valves 190277 a to 1902771 are provided betweenthe transfer chambers and the treatment chambers, so that the pressurein each treatment chamber can be controlled independently, and crosscontamination between the treatment chambers is prevented.

A substrate introduced into the transfer chamber 190260 from the loadingchamber 190262 is transferred to a predetermined treatment chamber by anarm type transfer means 190266 capable of rotating. The substrate istransferred from a certain treatment chamber to another treatmentchamber by the transfer means 190266. The transfer chambers 190260 and190261 are connected by the deposition treatment Chamber 190270 at whichthe substrate is delivered by the transfer means 190266 and a transfermeans 190267.

Each treatment chamber connected to the transfer chambers 190260 and190261 is maintained in a reduced pressure state. Accordingly, in theevaporation apparatus, deposition treatment of an EL layer iscontinuously performed without exposing the substrate to the room air. Adisplay panel in which formation of the EL layer is finished isdeteriorated due to moisture or the like in some cases. Accordingly, inthe evaporation apparatus, a sealing treatment chamber 190265 forperforming sealing treatment before exposure to the room air in order tomaintain quality is connected to the transfer chamber 190261. Since thesealing treatment chamber 190265 is under atmospheric pressure orreduced pressure near atmosphere pressure, an intermediate treatmentchamber 190264 is also provided between the transfer chamber 190261 andthe sealing treatment chamber 190265. The intermediate treatment chamber190264 is provided for delivering the substrate and buffering thepressure between the chambers.

An exhaust means is provided in the loading chamber, the unloadingchamber, the transfer chamber, and the deposition treatment chamber inorder to maintain reduced pressure in the chamber. As the exhaust means,various vacuum pumps such as a dry pump, a turbo-molecular pump, and adiffusion pump can be used.

In the evaporation apparatus of FIG. 88, the number of treatmentchambers connected to the transfer chambers 190260 and 190261 andstructures thereof can be combined as appropriate in accordance with astacked-layer structure of the EL element. An example of a combinationis described below.

In the heat treatment chamber 190268, degasification treatment isperformed by heating a substrate over which a lower electrode, aninsulating partition wall, or the like is formed. In the plasmatreatment chamber 190272, a surface of the lower electrode is treatedwith a rare gas or oxygen plasma. This plasma treatment is performed forcleaning the surface, stabilizing a surface state, or stabilizing aphysical or chemical state (e.g., a work function) of the surface.

The deposition treatment chamber 190269 is for forming an electrodebuffer layer which is in contact with one electrode of the EL element.The electrode buffer layer has a carrier injection property (holeinjection or electron injection) and suppresses generation of ashort-circuit or a black spot defect of the EL element. Typically, theelectrode buffer layer is formed of an organic-inorganic hybridmaterial, has a resistivity of 5×10⁴ to 1×10⁶ Ωcm, and is formed havinga thickness of 30 to 300 nm. Note that the deposition treatment chamber190271 is for forming a hole transporting layer.

A light-emitting layer in an EL element has a different structurebetween the case of emitting single color light and the case of emittingwhite light. A deposition treatment chamber in the evaporation apparatusis preferably provided depending on the structure. For example, whenthree kinds of EL elements each having a different light emission colorare formed in a display panel, it is necessary to form light-emittinglayers corresponding to respective light emission colors. In this case,the deposition treatment chamber 190270 can be used for forming a firstlight-emitting layer, the deposition treatment chamber 190273 can beused for forming a second light-emitting layer, and the depositiontreatment chamber 190274 can be used for forming a third light-emittinglayer. By using different deposition treatment chambers for respectivelight-emitting layers, cross contamination due to differentlight-emitting materials can be prevented, and throughput of thedeposition treatment can be improved.

Note that three kinds of EL elements each having a different lightemission color may be sequentially deposited in each of the depositiontreatment chambers 190270, 190273, and 190274. In this case, evaporationis performed by moving a shadow mask in accordance with a region to bedeposited.

When an EL element which emits white light is formed, the EL element isformed by vertically stacking light-emitting layers of different lightemission colors. In this case also, the element substrate can betransferred through the deposition treatment chambers sequentially toform each light-emitting layer. Alternatively, different light-emittinglayers can be formed continuously in the same deposition treatmentchamber.

In the deposition treatment chamber 190276, an electrode is formed overthe EL layer. The electrode can be formed by an electron beamevaporation method or a sputtering method, and preferably by aresistance heating evaporation method.

The element substrate in which formation of the electrode is finished istransferred to the sealing treatment chamber 190265 through theintermediate treatment chamber 190264. The sealing treatment chamber190265 is filled with an inert gas such as helium, argon, neon, ornitrogen, and a sealing substrate is attached to a side of the elementsubstrate where the EL layer is formed under the atmosphere to besealed. In a sealed state, a space between the element substrate and thesealing substrate may be filled with the inert gas or a resin material.The sealing treatment chamber 190265 is provided with a dispenser whichprovides a sealing material, a mechanical element such as an arm or afixing stage which fixes the sealing substrate to face the elementsubstrate, a dispenser or a spin coater which fills the chamber with aresin material, and the like.

FIG. 89 shows an internal structure of a deposition treatment chamber.The deposition treatment chamber is maintained in a reduced pressurestate. In FIG. 89, a space interposed between a top plate 190391 and abottom plate 190392 corresponds to an internal space of the chamber,which is maintained in a reduced pressure state.

One or a plurality of evaporation sources are provided in the treatmentchamber. This is because a plurality of evaporation sources arepreferably provided when a plurality of layers having differentcompositions are formed or when different materials are co-evaporated.In FIG. 89, evaporation sources 190381 a, 190381 b, and 190381 c areattached to an evaporation source holder 190380. The evaporation sourceholder 190380 is held by a multi-joint arm 190383. The multi-joint arm190383 allows the evaporation source holder 190380 to move within itsmovable range by stretching the joint. Alternatively, the evaporationsource holder 190380 may be provided with a distance sensor 190382 tomonitor a distance between the evaporation sources 190381 a to 190381 cand a substrate 190389 so that an optimal distance for evaporation maybe controlled. In this case, the multi-joint arm may be capable ofmoving toward upper and lower directions (Z direction) as well.

The substrate 190389 is fixed by using a substrate stage 190386 and asubstrate chuck 190387 together. The substrate stage 190386 may have astructure where a heater is incorporated so that the substrate 190389can be heated. The substrate 190389 is transferred by tightening thesubstrate chuck 190387 while being fixed to the substrate stage 190386.At the time of evaporation, a shadow mask 190390 provided with anopening corresponding to a deposition pattern can be used when needed.In this case, the shadow mask 190390 is provided between the substrate190389 and the evaporation sources 190381 a to 190381 c. The shadow mask190390 is fixed to the substrate 190389 in close contact with each otheror with a certain interval therebetween by a mask chuck 190388. Whenalignment of the shadow mask 190390 is needed, the alignment isperformed by arranging a camera in the treatment, chamber and providingthe mask chuck 190388 with a positioning means which slightly moves inX-Y-θ directions.

The evaporation sources 190381 a to 190381 c include an evaporationmaterial supply means which continuously supplies an evaporationmaterial to the evaporation source. The evaporation material supplymeans includes evaporation material supply sources 190385 a, 190385 b,and 190385 c, which are provided apart from the evaporation sources190381 a, 190381 b, and 190381 c, and a material supply pipe 190384which connects between the evaporation source and the evaporationmaterial supply source. Typically, the material supply sources 190385 ato 190385 c are provided corresponding to the evaporation sources 190381a to 190381 c. In FIG. 89, the material supply source 190385 acorresponds to the evaporation source 190381 a, the material supplysource 190385 b corresponds to the evaporation source 190381 b, and thematerial supply source 190385 c corresponds to the evaporation source190381 c.

As a method for supplying an evaporation material, an airflow transfermethod, an aerosol method, or the like can be employed. In an airflowtransfer method, impalpable powder of an evaporation material istransferred in airflow to the evaporation sources 190381 a to 190381 cby using an inert gas or the like. In an aerosol method, evaporation isperformed while material liquid in which an evaporation material isdissolved or dispersed in a solvent is transferred and aerosolized by anatomizer, and the solvent in the aerosol is vaporized. In each case, theevaporation sources 190381 a to 190381 c are provided with a heatingmeans, and a film is formed over the substrate 190389 by vaporizing theevaporation material transferred thereto.

In FIG. 89, the material supply pipe 190384 can be bent flexibly and isformed of a thin pipe which has enough rigidity not to be transformedeven under reduced pressure.

When an airflow transfer method or an aerosol method is employed, filmformation may be performed under atmospheric pressure or lower pressurein the deposition treatment chamber, and preferably performed under areduced pressure of 133 to 13300 Pa. The pressure can be adjusted whilean inert gas such as helium, argon, neon, krypton, xenon, or nitrogenfills the deposition treatment chamber or is supplied (and exhausted atthe same time) to the deposition treatment chamber. Note that anoxidizing atmosphere may be employed by introducing a gas such as oxygenor nitrous oxide in the deposition treatment chamber where an oxide filmis formed. Alternately, a reducing atmosphere may be employed byintroducing a gas such as hydrogen in the deposition treatment chamberwhere an organic material is deposited.

As another method for supplying an evaporation material, a screw may beprovided in the material supply pipe 190384 to continuously push theevaporation material toward the evaporation source.

With this evaporation apparatus, a film can be formed continuously withhigh uniformity even in the case of a large display panel. Since it isnot necessary to supply an evaporation material to the evaporationsource every time the evaporation material is ran out, throughput can beimproved.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

Embodiment Mode 17

In this embodiment mode, examples of electronic devices according to theinvention are described.

FIG. 90 shows a display panel module combining a display panel 900101and a circuit board 900111. The display panel 900101 includes a pixelportion 900102, a scan line driver circuit 900103, and a signal linedriver circuit 900104. The circuit board 900111 is provided with acontrol circuit 900112, a signal dividing circuit 900113, and the like,for example. The display panel 900101 and the circuit board 900111 areconnected to each other by a connection wiring 900114. An FPC or thelike can be used as the connection wiring.

In the display panel 900101, the pixel portion 900102 and part ofperipheral driver circuits (a driver circuit having a low operationfrequency among a plurality of driver circuits) may be formed over thesame substrate by using transistors, and another part of the peripheraldriver circuits (a driver circuit having a high operation frequencyamong the plurality of driver circuits) may be formed over an IC chip.Then, the IC chip may be mounted on the display panel 900101 by COG(Chip On Glass) or the like. Thus, the area of the circuit board 900111can be reduced, and a small display device can be obtained.Alternatively, the IC chip may be mounted on the display panel 900101 byusing TAB (Tape Automated Bonding) or a printed wiring board. Thus, thearea of the display panel 900101 can be reduced, and a display devicewith a narrower frame can be obtained.

For example, in order to reduce power consumption, a pixel portion maybe formed over a glass substrate by using transistors, and allperipheral circuits may be formed over an IC chip. Then, the IC chip maybe mounted on a display device by COG or TAB.

By the display panel module shown in FIG. 90, a television receiver canbe completed. FIG. 91 is a block diagram showing a main structure of atelevision receiver. A tuner 900201 receives a video signal and an audiosignal. The video signals are processed by an video signal amplifiercircuit 900202; a video signal processing circuit 900203 which convertsa signal output from the video signal amplifier circuit 900202 into acolor signal corresponding to each color of red, green, and blue; and acontrol circuit 900212 which converts the video signal into the inputspecification of a driver circuit. The control circuit 900212 outputs asignal to each of a scan line driver circuit 900210 and a signal linedriver circuit 900204. The scan line driver circuit 900210 and thesignal line driver circuit 900204 drive a display panel 900211. Whendigital drive is performed, a structure may be employed in which asignal dividing circuit 900213 is provided on the signal line side andan input digital signal is divided into m signals (m is a positiveinteger) to be supplied.

Among the signals received by the tuner 900201, an audio signal istransmitted to an audio signal amplifier circuit 900205, and an outputthereof is supplied to speaker 900207 through an audio signal processingcircuit 900206. A control circuit 900208 receives control information onreceiving station (receiving frequency) and volume from an input portion900209 and transmits signals to the tuner 900201 or the audio signalprocessing circuit 900206.

FIG. 92A shows a television receiver incorporated with a display panelmodule which is different from FIG. 91. In FIG. 92A, a display screen900302 incorporated in a housing 900301 is formed using the displaypanel module. Note that speakers 900303, an operation switch 900304, andthe like may be provided as appropriate.

FIG. 92B shows a television receiver in which only a display can becarried wirelessly. A battery and a signal receiver are incorporated ina housing 900312. The battery drives a display portion 900313 or aspeaker portion 900317. The battery can be repeatedly charged by acharger 900310. The charger 900310 can transmit and receive a videosignal and transmit the video signal to the signal receiver of thedisplay. The housing 900312 is controlled by an operation key 900316.Alternatively, the device shown in FIG. 92B can transmit a signal to thecharger 900310 from the housing 900312 by operating the operation key900316. That is, the device may be an image and audio interactivecommunication device. Further alternatively, by operating the operationkey 900316, a signal is transmitted to the charger 900310 from thehousing 900312, and another electronic device is made to receive asignal which can be transmitted from the charger 900310; thus, thedevice can control communication of another electronic device. That is,the device may be a general-purpose remote control device. The inventioncan be applied to the display portion 900313.

FIG. 93A shows a module combining a display panel 900401 and a printedwiring board 900402. The display panel 900401 may include a pixelportion 900403 provided with a plurality of pixels, a first scan linedriver circuit 900404; a second scan line driver circuit 900405, and asignal line driver circuit 900406 which supplies a video signal to aselected pixel.

The printed wiring board 900402 is provided with a controller 900407, acentral processing unit (CPU) 900408, a memory 900409, a power supplycircuit 9004010, an audio processing circuit 900411, atransmitting/receiving circuit 900412, and the like. The printed wiringboard 900402 and the display panel 900401 are connected by a flexibleprinted circuit (FPC) 900413. The flexible printed circuit (FPC) 900413may have a structure where a capacitor, a buffer circuit, or the like isprovided to prevent noise on power supply voltage or a signal, andincrease in rise time of a signal. Note that the controller 900407, theaudio processing circuit 900411, the memory 900409, the centralprocessing unit (CPU) 900408, the power supply circuit 900410, or thelike can be mounted to the display panel 900401 by using a COG (Chip OnGlass) method. By using a COG method, the size of the printed wiringboard 900402 can be reduced.

Various control signals are input and output through an interface (I/F)portion 900414 provided in the printed wiring board 900402. An antennaport 900415 for transmitting and receiving a signal to/from an antennais provided in the printed wiring board 900402.

FIG. 93B is a block diagram of the module shown in FIG. 93A. The moduleincludes a VRAM 900416, a DRAM 900417, a flash memory 900418, and thelike as the memory 900409. The VRAM 900416 stores data on an imagedisplayed on a panel, the DRAM 900417 stores video data or audio data,and the flash memory 900418 stores various programs.

The power supply circuit 900410 supplies electric power for operatingthe display panel 900401, the controller 900407, the central processingunit (CPU) 900408, the audio processing circuit 900411, the memory900409, and the transmitting/receiving circuit 900412. Note that thepower supply circuit 900410 may be provided with a current sourcedepending on a panel specification.

The central processing unit (CPU) 900408 includes a control signalgeneration circuit 900490, a decoded 900421, a register 900422, anarithmetic circuit 900423, a RAM 900424, an interface (I/F) portion900419 for the central processing unit (CPU) 900408, and the like.Various signals input to the central processing unit (CPU) 900408 via aninterface (I/F) portion 900414 are once stored in the register 900422,and subsequently input to the arithmetic circuit 900423, the decoder900421, or the like. The arithmetic circuit 900423 performs operationbased on the signal input thereto so as to designate a location to whichvarious instructions are sent. On the other hand, the signal input tothe decoder 900421 is decoded and input to the control signal generationcircuit 900420. The control signal generation circuit 900420 generates asignal including various instructions based on the signal input thereto,and transmits the signal to the location designated by the arithmeticcircuit 900423, specifically the memory 900409, thetransmitting/receiving circuit 900412, the audio processing circuit900411, and the controller 900407, for example.

The memory 900409, the transmitting/receiving circuit 900412, the audioprocessing circuit 900411, and the controller 900407 operate inaccordance with respective instructions. Hereinafter, the operation isbriefly described.

A signal input from an input means 900425 is transmitted to the centralprocessing unit (CPU) 900408 mounted to the printed wiring board via theinterface (I/F) portion 900414. The control signal generation circuit900420 converts image data stored in the VRAM 900416 into apredetermined format depending on the signal transmitted from the inputmeans 900425 such as a pointing device or a keyboard, and transmits theconverted data to the controller 900407.

The controller 900407 performs data processing of the signal includingthe image data transmitted from the central processing unit (CPU) 900408in accordance with the panel specification, and supplies the signal tothe display panel 900401. The controller 900407 generates an Hsyncsignal, a Vsync signal, a clock signal CLK, alternating voltage (ACCont), and a switching signal L/R based on power supply voltage inputfrom the power supply circuit 900410 or various signals input from thecentral processing unit (CPU) 900408, and supplies the signals to thedisplay panel 900401.

The transmitting/receiving circuit 900412 processes a signal which is tobe transmitted and received as an electric wave by an antenna 900428.Specifically, the transmitting/receiving circuit 900412 may include ahigh-frequency circuit such as an isolator, a band pass filter, a VCO(voltage controlled oscillator), an LPF (low pass filter), a coupler, ora balun. A signal including audio information among signals transmittedand received by the transmitting/receiving circuit 900412 is transmittedto the audio processing circuit 900411 in accordance with an instructionfrom the central processing unit (CPU) 900408.

The signal including the audio information which is transmitted inaccordance with the instruction from the central processing unit (CPU)900408 is demodulated into an audio signal by the audio processingcircuit 900411 and transmitted to a speaker 900427. An audio signaltransmitted from a microphone 900426 is modulated by the audioprocessing circuit 900411 and transmitted to the transmitting/receivingcircuit 900412 in accordance with an instruction from the centralprocessing unit (CPU) 900408.

The controller 900407, the central processing unit (CPU) 900408, thepower supply circuit 900410, the audio processing circuit 900411, andthe memory 900409 can be mounted as a package of this embodiment mode.

It is needless to say that this embodiment mode is not limited to atelevision receiver and can be applied to various uses, such as amonitor of a personal computer, and especially as a large display mediumsuch as an information display board at the train station, the airport,or the like, or an advertisement display board on the street.

Next, a structure example of a mobile phone according to the inventionis described with reference to FIG. 94.

A display panel 900501 is detachably incorporated in a housing 900530.The shape or the size of the housing 900530 can be changed asappropriate in accordance with the size of the display panel 900501. Thehousing 900530 which fixes the display panel 900501 is fitted in aprinted wiring board 900531 to be assembled as a module.

The display panel 900501 is connected to the printed wiring board 900531through an FPC 900513. The printed wiring board 900531 is provided witha speaker 900532, a microphone 900533, a transmitting/receiving circuit900534, and a signal processing circuit 900535 including a CPU, acontroller, and the like. Such a module, an input means 900536, and abattery 900537 are combined and stored in a housing 900539. A pixelportion of the display panel 900501 is provided to be seen from anopening window formed in the housing 900539.

In the display panel 900501, the pixel portion and part of peripheraldriver circuits (a driver circuit having a low operation frequency amonga plurality of driver circuits) may be formed over the same substrate byusing transistors, and another part of the peripheral driver circuits (adriver circuit having a high operation frequency among the plurality ofdriver circuits) may be formed over an IC chip. Then, the IC chip may bemounted on the display panel 900501 by COG (Chip On Glass).Alternatively, the IC chip may be connected to a glass substrate byusing TAB (Tape Automated Bonding) or a printed wiring board. With sucha structure, power consumption of a display device can be reduced, andoperation time of the mobile phone per charge can be extended. Further,reduction in cost of the mobile phone can be realized.

In a mobile phone shown in FIG. 95, a main body (A) 900601 provided withoperation switches 900604, a microphone 900605, and the like isconnected to a main body (B) 900602 provided with a display panel (A)900608, a display panel (B) 900609, a speaker 900606, and the like byusing a hinge 900610 so that the mobile phone can be opened and closed.The display panel (A) 900608 and the display panel (B) 900609 are placedin a housing 900603 of the main body (B) 900602 together with a circuitboard 900607. Pixel portions of the display panel (A) 900608 and thedisplay panel (B) 900609 are arranged to be seen from an opening windowformed in the housing 900603.

Specifications of the display panel (A) 900608 and the display panel (B)900609, such as the number of pixels, can be set as appropriate inaccordance with functions of a mobile phone 900600. For example, thedisplay panel (A) 900608 used as a main screen and the display panel (B)900609 used as a sub-screen can be combined.

A mobile phone according to this embodiment mode can be changed invarious modes depending on functions or applications thereof. Forexample, it may be a camera-equipped mobile phone by incorporating animaging element in the hinge 900610. When the operation switches 900604,the display panel (A) 900608, and the display panel (B) 900609 areplaced in one housing, the aforementioned effects can be obtained.Further, a similar effect can be obtained when the structure of thisembodiment mode is applied to an information display terminal equippedwith a plurality of display portions.

The invention can be applied to various electronic devices.Specifically, the invention can be applied to a display portion of anelectronic device. Examples of such electronic devices include camerassuch as a video camera and a digital camera, a goggle-type display, anavigation system, an audio reproducing device (such as car audiocomponents and audio components), a computer, a game machine, a portableinformation terminal (such as a mobile computer, a mobile phone, amobile game machine, and an electronic book), and an image reproducingdevice provided with a recording medium (specifically, a device whichreproduces a recording medium such as a digital versatile disc (DVD) andhas a display for displaying the reproduced image).

FIG. 96A shows a display, which includes a housing 900711, a supportbase 900712, a display portion 900713, and the like.

FIG. 96B shows a camera, which includes a main body 900721, a displayportion 900722, an image receiving portion 900723, operation keys900724, an external connection port 900725, a shutter button 900726, andthe like.

FIG. 96C shows a computer, which includes a main body 900731, a housing900732, a display portion 900733, a keyboard 900734, an externalconnection port 900735, a pointing device 900736, and the like.

FIG. 96D shows a mobile computer, which includes a main body 900741, adisplay portion 900742, a switch 900743, operation keys 900744, aninfrared port 900745, and the like.

FIG. 96E shows a portable image reproducing device having a recordingmedium (e.g., a DVD reproducing device), which includes a main body900751, a housing 900752, a display portion A 900753, a display portionB 900754, a recording medium (e.g., DVD) reading portion 900755,operation keys 900756, a speaker portion 900757, and the like. Thedisplay portion A 900753 can mainly display image information, and thedisplay portion B 900754 can mainly display text information.

FIG. 96F shows a goggle-type display, which includes a main body 900761,a display portion 900762, an earphone 900763, a support portion 900764,and the like.

FIG. 96G shows a portable game machine, which includes a housing 900771,a display portion 900772, a speaker portion 900773, operation keys900774, a recording medium insert portion 900775, and the like. Theportable game machine in which the display device in the invention isused for the display portion 900772 can express bright colors.

FIG. 96H shows a digital camera having a television reception function,which includes a housing 900781, a display portion 900782, operationkeys 900783, a speaker 900784, a shutter button 900785, an imagereceiving portion 900786, an antenna 900787, and the like.

As shown in FIGS. 96A to 96H, the electronic device according to theinvention includes a display portion for displaying some kind ofinformation. The electronic device according to the invention has lowpower consumption, and can drive with a battery for a long time.Further, a moving image without motion blur can be displayed. Moreover,a manufacturing method is simple, and manufacturing cost can be reduced.

Next, application examples of a semiconductor device according to theinvention are described.

FIG. 97 shows an example where the semiconductor device according to theinvention is incorporated in a constructed object. FIG. 97 shows ahousing 900810, a display portion 900811, a remote control device 900812which is an operation portion, a speaker portion 900813, and the like.The semiconductor device according to the invention is incorporated inthe constructed object as a wall-hanging type and can be providedwithout requiring a large space.

FIG. 98 shows another example where the semiconductor device accordingto the invention is incorporated in a constructed object. A displaypanel 900901 is incorporated with a prefabricated bath 900902, and aperson who takes a bath can view the display panel 900901. The displaypanel 900901 has a function of displaying information by an operation bythe person who takes a bath; and a function of being used as anadvertisement or an entertainment means.

Note that the semiconductor device according to the invention can beprovided not only to a side wall of the prefabricated bath 900902 asshown in FIG. 98, but also to various places. For example, thesemiconductor device can be incorporated with part of a mirror, abathtub itself, or the like. At this time, a shape of the display panel900901 may be changed in accordance with a shape of the minor or thebathtub.

FIG. 99 shows another example where the semiconductor device accordingto the invention is incorporated in a constructed object. A displaypanel 901002 is bent and attached to a curved surface of a column-shapedobject 901001. Note that here, a utility pole is described as thecolumn-shaped object 901001.

The display panel 901002 shown in FIG. 99 is provided at a positionhigher than a human viewpoint. When the same images are displayed on thedisplay panels 901002 provided in constructed objects which standtogether in large numbers outdoors, such as utility poles, advertisementcan be performed to an unspecified number of viewers. Since it is easyfor the display panel 901002 to display the same images and instantlyswitch images by external control, highly efficient information displayand advertisement effect can be realized. When provided withself-luminous display elements, the display panel 901002 can be usefulas a highly visible display medium even at night. When the display panel901002 is provided in the utility pole, a power supply means for thedisplay panel 901002 can be easily obtained. In an emergency such asdisaster, the display panel 901002 can also be used as a means totransmit correct information to victims rapidly.

Note that an example of the display panel 901002 includes a displaypanel in which a switching element such as an organic transistor isprovided over a film-shaped substrate and a display element is driven soas to display an image.

Note that in this embodiment mode, a wall, a column-shaped object, and aprefabricated bath are shown as examples of constructed objects;however, this embodiment mode is not limited thereto, and variousconstructed objects can be provided with the semiconductor deviceaccording to the invention.

Next, examples where the semiconductor device according to the inventionis incorporated with a moving object are described.

FIG. 100 shows an example where the semiconductor device according tothe invention is incorporated with a car. A display panel 901101 isincorporated with a car body 901102, and can display an operation of thecar body or information input from inside or outside the car body ondemand. Note that a navigation function may be provided.

The semiconductor device according to the invention can be provided notonly to the car body 901102 as shown in FIG. 100, but also to variousplaces. For example, the semiconductor device can be incorporated with aglass window, a door, a steering wheel, a gear shift, a seat, arear-view minor, and the like. At this time, a shape of the displaypanel 901101 may be changed in accordance with a shape of an objectprovided with the semiconductor device.

FIGS. 101A and 101B show examples where the semiconductor deviceaccording to the invention is incorporated with a train car.

FIG. 101A shows an example where a display panel 901202 is provided inglass of a door 901201 in a train car, which has an advantage comparedwith a conventional advertisement using paper in that labor cost forchanging an advertisement is not necessary. Since the display panel901202 can instantly switch images displaying on a display portion by anexternal signal, images on the display panel can be switched in everytime period when types of passengers on the train are changed, forexample. Thus, more effective advertisement effect can be realized.

FIG. 101B shows an example where the display panels 901202 are providedto a glass window 901203 and a ceiling 901204 as well as the glass ofthe door 901201 in the train car. In this manner, the semiconductordevice according to the invention can be easily provided to a placewhere a semiconductor device has been difficult to be providedconventionally; thus, effective advertisement effect can be obtained.Further, the semiconductor device according to the invention caninstantly switch images displayed on a display portion by an externalsignal; thus, cost and time for changing an advertisement can bereduced, and more flexible advertisement management and informationtransmission can be realized.

Note that the semiconductor device according to the invention can beprovided not only to the door 901201, the glass window 901203, and theceiling 901204 as shown in FIGS. 101A and 101B, but also to variousplaces. For example, the semiconductor device can be incorporated with astrap, a seat, a handrail, a floor, and the like. At this time, a shapeof the display panel 901202 may be changed in accordance with a shape ofan object provided with the semiconductor device.

FIGS. 102A and 102B show an example where the semiconductor deviceaccording to the invention is incorporated with a passenger airplane.

FIG. 102A shows a shape of a display panel 901302 attached to a ceiling901301 above a seat of the passenger airplane when the display panel901302 is used. The display panel 901302 is incorporated with theceiling 901301 with a hinge portion 901303, and a passenger can view thedisplay panel 901302 by stretching of the hinge portion 901303. Thedisplay panel 901302 has a function of displaying information by anoperation by the passenger and a function of being used as anadvertisement or an entertainment means. As shown in FIG. 102B, thehinge portion is bent and the display panel is stored in the ceiling901301, so that safety in taking-off and landing can be assured. Notethat in an emergency, the display panel can also be used as aninformation transmission means and an evacuation light by lighting adisplay element in the display panel.

The semiconductor device according to the invention can be provided notonly to the ceiling 901301 as shown in FIGS. 102A and 102B, but also tovarious places. For example, the semiconductor device can beincorporated with a seat, a table attached to a seat, an armrest, awindow, and the like. A large display panel which a plurality of peoplecan view may be provided at a wall of an airframe. At this time, a shapeof the display panel 901302 may be changed in accordance with a shape ofan object provided with the semiconductor device.

Note that in this embodiment mode, bodies of a train car, a car, and anairplane are shown as moving objects; however, the invention is notlimited thereto, and the semiconductor device according to the inventioncan be provided to various objects such as a motorcycle, an four-wheeldrive car (including a car, a bus, and the like), a train (including amonorail, a railroad car, and the like), and a vessel. Since thesemiconductor device according to the invention can instantly switchimages displayed on a display panel in a moving object by an externalsignal, the moving object provided with the semiconductor deviceaccording to the invention can be used as an advertisement display boardfor an unspecified number of customers, an information display board indisaster, and the like.

Although this embodiment mode is described with reference to variousdrawings, the contents (or part of the contents) described in eachdrawing can be freely applied to, combined with, or replaced with thecontents (or part of the contents) described in another drawing.Further, much more drawings can be formed by combining each part withanother part in the above-described drawings.

The contents (or part of the contents) described in each drawing in thisembodiment mode can be freely applied to, combined with, or replacedwith the contents (or part of the contents) described in a drawing inanother embodiment mode. Further, much more drawings can be formed bycombining each part in each drawing in this embodiment mode with part ofanother embodiment mode.

This embodiment mode shows examples of embodying, slightly transforming,partially modifying, improving, describing in detail, or applying thecontents (or part of the contents) described in other embodiment modes,an example of related part thereof, or the like. Therefore, the contentsdescribed in other embodiment modes can be freely applied to, combinedwith, or replaced with this embodiment mode.

This application is based on Japanese Patent Application serial No.2006-328670 filed with Japan Patent Office on Dec. 5, 2006, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. (canceled)
 2. A display device comprising: adisplay element; and a backlight, wherein the display element has afirst period for analyzing gray scales of an image to obtain a data ofthe image, and a second period for displaying the image, wherein atransmittance of the display element is higher in the second period thanin the first period, and wherein a luminance of the backlight is lowerin the second period than in the first period.
 3. The display deviceaccording to claim 2, further comprising: a control circuit controllingthe luminance of the backlight.
 4. The display device according to claim2, wherein in the second period, a product of the transmittance of thedisplay element and the luminance of the backlight is saturated in agray scale region at high gray scale side, and wherein the gray scaleregion is determined so that a number of the data of the image includedin the gray scale region is equal to or less than 1/10 of a total numberof the data of the image.
 5. The display device according to claim 2,wherein in the second period, a product of the transmittance of thedisplay element and the luminance of the backlight has a gradient in agray scale region at high gray scale side, and wherein the gray scaleregion is determined so that a number of the data of the image includedin the gray scale region is equal to or less than 1/10 of a total numberof the data of the image.
 6. The display device according to claim 4,wherein the gray scale region is changed by varying the luminance of thebacklight.
 7. The display device according to claim 5, wherein the grayscale region is changed by varying the luminance of the backlight. 8.The display device according to claim 2, wherein the display element hasan R pixel, a G pixel, a B pixel, and a W pixel.
 9. The display deviceaccording to claim 2, wherein the display element has a transistorincluding an oxide semiconductor comprising In, Ga, and Zn.
 10. Thedisplay device according to claim 2, wherein the display element is aliquid crystal element.
 11. A driving method of a display devicecomprising a display element and a backlight, the driving methodcomprising the steps of: analyzing gray scales of an image to obtain adata of the image in a first period; and displaying the image in asecond period, wherein a transmittance of the display element is higherin the second period than in the first period, and wherein a luminanceof the backlight is lower in the second period than in the first period.12. The driving method of a display device according to claim 11, thedisplay device further comprising: a control circuit controlling theluminance of the backlight.
 13. The driving method of a display deviceaccording to claim 11, wherein in the second period, a product of thetransmittance of the display element and the luminance of the backlightis saturated in a gray scale region at high gray scale side, and whereinthe gray scale region is determined so that a number of the data of theimage included in the gray scale region is equal to or less than 1/10 ofa total number of the data of the image.
 14. The driving method of adisplay device according to claim 11, wherein in the second period, aproduct of the transmittance of the display element and the luminance ofthe backlight has a gradient in a gray scale region at high gray scaleside, and wherein the gray scale region is determined so that a numberof the data of the image included in the gray scale region is equal toor less than 1/10 of a total number of the data of the image.
 15. Thedriving method of a display device according to claim 13, wherein thegray scale region is changed by varying the luminance of the backlight.16. The driving method of a display device according to claim 14,wherein the gray scale region is changed by varying the luminance of thebacklight.
 17. The driving method of a display device according to claim11, wherein the display element has an R pixel, a G pixel, a B pixel,and a W pixel.
 18. The driving method of a display device according toclaim 11, wherein the display element has a transistor including anoxide semiconductor comprising In, Ga, and Zn.
 19. The driving method ofa display device according to claim 11, wherein the display element is aliquid crystal element.